Which principal part is this verb form?

(have) looked

a. past

b. present

c. present participle

d. past participle

The correct answer is Present participle

The correct answer is D, past participle. That kind of participle is used in tenses such as the one you mentioned - past perfect.

A isn't correct because past would be just 'looked', without that 'have' that you've mentioned.

B would be look/looks.

C would be - looking.