It is only . . . . . . . . the end of the book that we find out who the father really was. A. about B. in C. of D. at

The correct answer is D. at.

The preposition “at” is used to indicate a specific point in time. In this case, the point in time is the end of the book. The other prepositions are not as specific. “About” can indicate a general time period, “in” can indicate a longer period of time, and “of” is not typically used to indicate time.

Here are some examples of how the preposition “at” is used to indicate a specific point in time:

• I arrived at the airport at 10:00 AM.
• The meeting is at 2:00 PM.
• The party starts at 8:00 PM.

In the sentence “It is only at the end of the book that we find out who the father really was,” the preposition “at” indicates that the information about the father’s identity is revealed at a specific point in time, which is the end of the book.