Sunil employed my friend only when I agreed to avouch for his honesty.

The word "avouch" means to affirm or assert something. In the given sentence, it can be understood that only after the narrator affirms the honesty of his friend did Sunil employ him. 

Option D is a direct synonym of the word and would be the most appropriate choice. 

Option A would not fit in the sentence and can be easily eliminated. Options B and C are close and convey a similar sense but do not convey the same message as Avouch. 

Therefore, Option D is the correct answer.