Check my answer please.

Persian LearnerMy answer is #4. Am I right?

No.

Persian LearnerIf not, why?

Is the phrase active or passive?

Persian LearnerAm I right? If not, why?

No. "take part" is not used in the passive voice (part (was) taken in ...), so "taken" won't work. You need "taking". They were taking part in .... "were" is omitted, thus forming a reduced clause.

CJ

Persian LearnerHi.My answer is #4. Am I right? If not, why?

They were taking part in the programme. (It means that they actively, not passively, were taking part in it; note that "They"/"those"is a subject here, not an object.")

By the way, in my opinion, the verb "to take part in" cannot be used in the passive, hence "taken" is not right here.

AnonymousBy the way, in my opinion, the verb "to take part in" cannot be used in the passive,

The meeting was taken part in by thousands of adherents.

Not the best choice, possibly, but quite feasible.

Mister MicawberThe meeting was taken part in by thousands of adherents.Not the best choice, possibly, but quite feasible.

Yes, but what I meant was not the abstract nouns like 'meeting' but namely people, here 'they/those'. And I wonder whether nouns/pronouns denoting people could be used in passive constructions with the verb phrase 'to be taken part in'.

Anonymouswhether nouns/pronouns denoting people could be used in passive constructions with the verb phrase 'to be taken part in'

If that were possible it would suggest that we can (in the active voice) "take part in (someone)". I'd call this a big "No".