Fill in the blank in the given sentence by choosing the correct option. Please do not forget to submit your application______ time.

The correct answer is C) in. 'In time' means early enough, before a deadline or before something is needed. So 'submit your application in time' means submit it before the cutoff.

Why the others fail:

• by: 'by' fits a deadline ('submit by Friday'), but not this bare phrase. The idiom you need is 'by the time', which means something else.
• of: 'of time' is not an English idiom for timeliness, so the sentence becomes ungrammatical.
• with: 'with time' means gradually or eventually, as in 'with time, things improve'. That changes the meaning and does not fit a submission deadline.

Core concept: fixed prepositional phrases such as in time, on time, at the time and by the time. These are idioms, so they have to be learned as units rather than derived from rules. This sits under vocabulary and usage in the English section, and the same precision matters in Mains for official drafting and notices.

Trap: choosing 'on time' by reflex. 'On time' means punctual for a scheduled event, such as a train arriving on time. 'In time' means early enough to meet a deadline.