Explanation:

This question asks us to choose the most appropriate preposition to complete the sentence 'It was one ................. the tower clock.' The sentence is likely referring to the time being indicated or shown by the tower clock. Let's analyze the meaning and usage of each preposition: **Option (A): on** 'On' typically indicates a position on a surface (e.g., 'The book is on the table') or a specific day/date (e.g., 'on Monday'). If we say 'It was one on the tower clock,' it would imply that the number 'one' was physically located on the clock's surface, which is not the intended meaning of telling time. While a clock face has numbers 'on' it, the phrase 'one on the clock' is less common than 'one by the clock' when referring to the time indicated. **Option (B): by** 'By' can indicate the agent or means by which something is done, or it can mean 'according to' or 'as indicated by'. For example, 'The time is 5 o'clock by my watch.' In this context, 'It was one by the tower clock' means 'It was one o'clock according to the time shown on the tower clock.' This is a standard and correct way to indicate the source or authority for the time being stated. This preposition perfectly fits the intended meaning. **Option (C): with** 'With' indicates accompaniment, possession, or the use of an instrument. For example, 'I went with my friend' or 'He cut the paper with scissors.' 'It was one with the tower clock' does not make grammatical or semantic sense in the context of telling time. It implies association rather than indication of time. **Option (D): in** 'In' typically indicates a position inside something (e.g., 'The keys are in the drawer') or a period of time (e.g., 'in the morning'). 'It was one in the tower clock' would imply that the time 'one' was somehow *inside* the clock mechanism, which is illogical. It does not convey the meaning of time being indicated by the clock. **Conclusion:** When referring to the time shown or indicated by a clock or watch, the preposition 'by' is the most appropriate and grammatically correct choice. It signifies 'according to' or 'as per' the time source. Therefore, option (B) is the correct answer.