Answer:
The correct answer is C. singers'.
Step-by-step explanation:
When it comes to the possessive form of nouns, the general rule of thumb is to add an apostrophe (') after the noun, and add the suffix -s if the noun is singular; if the noun is plural, we just add the apostrophe to show possession.
In the example above, given that there is more than one singer in the group, we have to use the plural possessive form of the noun singers, which is singers'.
If there were only one singer, then option A would be correct. Option B doesn't show any possession, and therefore is incorrect in either case.