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 C is correct as the subject is singular and yet indicates the use of present perfect tense. Has is used as an auxiliary verb here, the main verb is buy which must be used in its third form.

 The other options are wrong as the present tense cannot be used. 

The correct answer is C)has/ bought