(a) Circumstances for State Government to File an Appeal Against Acquittal in Sessions Court: Under Section 378 of the Criminal Procedure Code, 1973, the State Government has the power to file an appeal against an order of acquittal. Specifically, regarding an acquittal by a Sessions Court:

1. Direction to Public Prosecutor: The State Government may direct the Public Prosecutor to present an appeal to the High Court from an original or appellate order of acquittal passed by any Court other than a High Court (which includes a Sessions Court).
2. Leave of the High Court: A crucial condition is that such an appeal cannot be entertained by the High Court unless the State Government (or the Central Government, in cases investigated by the Delhi Special Police Establishment or relating to an offence under any Central Act) obtains 'leave of the High Court' to file such an appeal. This means the High Court must first grant permission after considering the grounds for appeal.
3. Time Limit: There are specific periods of limitation for filing such appeals and for seeking leave to appeal (e.g., 90 days for an appeal from an order of acquittal, and 30 days for an application for leave to appeal, from the date of the order of acquittal).

Thus, the State Government can file an appeal against an acquittal by a Sessions Court, but it must first obtain the High Court's permission, and the appeal must be filed within the prescribed time limits.

(b) Powers of Magistrate to Pass Sentences (Section 29, CrPC): Magistrates have varying powers to pass sentences, depending on their class:

1. Chief Judicial Magistrate (CJM) / Chief Metropolitan Magistrate (CMM): Can pass any sentence authorized by law, except a sentence of death or of imprisonment for life or of imprisonment for a term exceeding seven years.
2. Metropolitan Magistrate (MM) / Judicial Magistrate First Class (JMFC): Can pass a sentence of imprisonment for a term not exceeding three years, or of fine not exceeding ten thousand rupees, or of both.
3. Judicial Magistrate Second Class (JMFC): Can pass a sentence of imprisonment for a term not exceeding one year, or of fine not exceeding five thousand rupees, or of both.

Magistrates can also award additional imprisonment in default of payment of fine, subject to certain limits (e.g., not exceeding one-fourth of the maximum imprisonment for the offence or one-fourth of the Magistrate's power, whichever is less).

(c) Action for Recovery of Fine in Case of Default: When an offender defaults in paying a fine, the court can take the following actions (Sections 64-70 IPC, Sections 30, 421, 424 CrPC):

1. Imprisonment in Default of Fine: The court can sentence the offender to imprisonment for a certain period in default of payment of the fine. This imprisonment is in addition to any substantive sentence awarded. If the fine is subsequently paid, the default imprisonment ceases.
2. Warrant for Levy of Fine (Section 421 CrPC): The court may issue a warrant for the levy of the amount by attachment and sale of any movable property belonging to the offender.
3. Warrant to Collector: The court may issue a warrant to the Collector of the district, authorizing him to realize the amount as arrears of land revenue from the movable or immovable property, or both, of the offender.
4. Attachment of Property (Section 424 CrPC): If the offender has absconded or disposed of property to avoid payment, the court can order the attachment of any property belonging to the offender.

The court typically prioritizes recovery through attachment and sale of property, and the default imprisonment is usually a secondary measure or runs concurrently until the fine is paid or recovered.

(d) Whether Magistrate Can Commit the Case to Court of Sessions After Commencement of Trial? Yes, a Magistrate can commit a case to the Court of Session even after the commencement of inquiry or trial, under the provisions of Section 323 of the Criminal Procedure Code, 1973.

Explanation: Section 323 CrPC provides that if, in any inquiry into an offence or a trial before a Magistrate, it appears to him at any stage of the proceedings before signing judgment that the case is one which ought to be tried by the Court of Session, or that he is not competent to award adequate punishment, he shall record his reasons and commit the case to the Court of Session. When committing the case, the Magistrate is required to recall any witnesses who may have been examined and proceed to re-examine them and resummons and re-hear the witnesses.

This provision is a safeguard to ensure that serious cases, or cases where the Magistrate's sentencing powers are insufficient to deliver appropriate justice, are tried by the higher court with broader jurisdiction and sentencing powers. It allows for a change in the forum of trial even after the proceedings have begun, preventing a miscarriage of justice due to an initial misassessment of the case's gravity or the Magistrate's limitations.