Answer:
B. After the band performed, a number of people said that their singing was at a professional level.
Explanation:
Since based on the context we don't know how many singers there were in the band, we can't choose a singular pronoun here. The singular pronouns mentioned in the options are her, its, and his or her, meaning that all of those options are incorrect. Because there may be more than one singer in the band, we must use the plural possessive pronoun their to refer to all of the singers in this band.
