Fill in the blank with the most appropriate word.
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 C is correct as last week indicates the use of simple past tense. Also, be is used as an ordinary and a main verb here.
The other options are wrong as no other tense can be used here.
The correct answer is C) was