They will sing a song.

A song was sung by them

A song is sung by them

A song will be sung by them

A song should be sung by them

None of the above

Explanation

They will sing a song means وہ ایک گانا گائیں گے

Future Indefinite Tense

Active Voice Formula: Subject + Will/Shall + Verb (Ist form) + Object + (.)
Passive Voice Formula: Convert Object to Subject + Will/Shall + Be + Verb (3rd form) + By + Convert Subject to Object + Remaining + (.)