Step-by-step explanation:
The correct options that use present participial adjectives and progressive tenses are:
a) They are singing in the choir.
c) They are singing a song.
In option a), "singing" is a present participle that functions as part of the progressive tense. It indicates an ongoing action of singing in the present.
In option c), "singing" is also a present participle, but it functions as a present participial adjective modifying the noun "song." It describes the type of song they are currently singing.
On the other hand, options b) and d) are not grammatically correct. "Sung" in option b) is a past participle, not a present participle, and does not fit with the progressive tense. In option d), "sung" is used as a past participle but is not correct in this context.