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

Answer: Option B

Vow : a solemn promise.
Affirm : state emphatically or publicly.
Pay : give (someone) money that is due for work done, goods received, or a debt incurred.
Guarantee : something that ensures a particular outcome.
Avouch : affirm or assert.

Synonym of Avouch is Affirm