Monday is the first day . . . . . . . the week.

The correct answer is Option A: of.

The correct sentence is: Monday is the first day of the week.

Explanation of the Grammar Rule:
The preposition "of" is used to show a relationship between a part and a whole or to indicate belonging. In this sentence, Monday is identified as the first day belonging to the week.

Why the Other Options Are Incorrect:
Option B: after — "After" indicates something that comes later in time or order. It does not fit the intended meaning of the sentence.

Option C: during — "During" means throughout or within a period of time. It is not appropriate for identifying the first day of a week.

Option D: before — "Before" indicates something that comes earlier than another time or event. It does not correctly express the relationship between Monday and the week.

Remember for Exams:
Use "of" to express the relationship between a part and a whole, as in "the first day of the week" or "the capital of India."