Fill in the blanks with the most appropriate words.
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 B is correct as returned indicates the use of past continuous tense. Also, the subject is you.
The other options are wrong as no other tense can be used here.
The correct answer is B) were/doing