Fill in the blank with the most appropriate word.
I _____ understand what you are saying.
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 gives is used in the formation of tenses, moods, tones, etc.
Option B understand indicates the use of present simple tense. Also, the use of not indicates the need of the helping verb do.
The other options are wrong as no other tense other than present simple tense can be used.
The correct answer is B)don't