A verb is used to express an occurrence, action, or state. It is the grammatical center of the predicate in a sentence.

An auxiliary verb is a verb which is used in the formation of tenses, moods, tones, etc.

Option A is correct as how long and has indicate the use of past participle. Also, be is used as an ordinary verb here.

 The other options are wrong as no other tense can be used here.

The correct answer is A) been