Explanation:

The role of the Comptroller and Auditor General (CAG) of India is constitutionally defined to audit the accounts of the Union and State governments, ensuring financial accountability and transparency. - **Option A (Guardian of the Public Purse):** This is the most accurate and widely recognized description of the CAG's role. The CAG acts as the protector of public funds, ensuring that government expenditure is in accordance with the law and that public money is used efficiently and effectively. This term emphasizes the CAG's oversight function in safeguarding the national exchequer. - **Option B (Keeper of the Public Purse):** While 'keeper' suggests a role in safekeeping, it is less formal and precise than 'guardian'. The CAG's function is not merely to keep records but to actively scrutinize and report on financial management. - **Option C (Custodian of the Public Purse):** 'Custodian' implies holding something in trust or for safekeeping. However, the CAG does not physically hold or manage the public purse; rather, the CAG audits how the executive manages it. 'Guardian' better conveys the active oversight and protective role. - **Option D (Trustee of the Public Purse):** A trustee manages assets on behalf of beneficiaries. The CAG does not manage the public purse; instead, the CAG audits the government's management of the public purse. Therefore, 'trustee' is an incorrect description of the CAG's function.