A) they

(watch) TV last night

They watched TV last night

They watched TV last night.

Explanation:

I couldn't find the complete form to this question online. But since it has an adverb of time: "last night", we can deduce that the verb tense to which the verb "watch" must be passed is a simple past tense.

These types of adverbs tell us just when the action of our sentence happens.

While the simple past tense serves to indicate something that happened at a specific time in the past, and there it ended. In this case, it happened last night.

Next I will write the verb conjugated in the past simple for each of the people in English:

I watched you watched he/she/it watched we watched you watched they watched