Fill in the blank with the most appropriate word.
A liar _____ when he speaks the truth.
A verb is used to express an occurrence, action, or state. It is the grammatical centre of the predicate in a sentence.
An auxiliary verb is a verb which is used in the formation of tenses, moods, tones, etc.
Option B is correct as speaks indicates the use of simple present tense. The sentence is given in the passive voice.
The other options are wrong as no other tense can be used here.
The correct answer is B) isn't believed