Select the most appropriate option to fill in the blank.

I don’t think your sister is enjoying the party. She ______ at her watch for the last half an hour.

has been looking

The correct option to fill in the blank is has been looking.

The sentence is about the speaker's sister, who has been looking at her watch for the past half an hour. The word "has been looking" is the present perfect continuous tense of the verb "to look", which means to keep looking at something for a period of time. In this context, the speaker is saying that the sister has been looking at her watch for a long time, which suggests that she is not enjoying the party.

The other options are not as appropriate.

• Had looked is the past perfect tense of the verb "to look", which means to look at something before a certain time in the past. This is not the most appropriate option because it does not suggest that the sister is still looking at her watch.
• Has looked is the simple present tense of the verb "to look", which means to look at something once. This is not the most appropriate option because it does not suggest that the sister has been looking at her watch for a long time.
• Had been looking is the past perfect continuous tense of the verb "to look", which means to keep looking at something for a period of time before a certain time in the past. This is not the most appropriate option because it suggests that the sister stopped looking at her watch at some point in the past.