To solve this question, the user needs to know the correct preposition to use in the given sentence.

The correct preposition to use in this sentence is "about". The correct sentence is "It's about time you told him the truth!"

Explanation for each option:

A. "On" is not the correct preposition to use in this sentence. Using "on" would result in an incorrect sentence.

B. "About" is the correct preposition to use in this sentence. Using "about" would result in the correct sentence.

C. "In" is not the correct preposition to use in this sentence. Using "in" would result in an incorrect sentence.

D. "For" is not the correct preposition to use in this sentence. Using "for" would result in an incorrect sentence.

Therefore, the answer is: B. about.