Answer:
Step-by-step explanation:
Donizetti's Gemma di Vergy, as well as many other belcanto operas of the first half of the 19th century, is infrequently produced today, mostly on account of requiring singers with a virtuosic vocal ability in all of the roles.
A.on account of requiring singers
B.on account of their requiring a singer
C.because it requires singers
D.because of it requiring singers
E.being that it requires a singer
The use of "on account of" with a gerund ("requiring") is wordy and unidiomatic.Option C conveys the meaning in the simplest form and hence is the correct option. All other options are awkwardly wordy.