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 the sentence is given in the passive voice and indicates the use of the past participle form of the verb.
The other options are wrong as no other tense can be used here.
The correct answer is C) spoken.