The correct answer is (b) Seated.
The sentence is in the past tense, and the verb needs to agree with the subject "people." "Seated" is the correct past participle form of the verb "to seat," and it correctly indicates that the people were placed in their seats before the action or event that is being referred to in the sentence.
Option (a) Seating is a present participle, which would indicate an ongoing action, so it does not fit in this context. Option (c) Seat is a noun, not a verb, so it does not work as the answer. Option (d) Been seated is the past perfect tense, which would indicate an action that occurred before another past action, but it does not fit in this context either, as there is no other past action mentioned in the sentence