I was confused between C and D. But, I pick up D for its continuity of sense of tense.
C is correct usage but it should not be a general statement. Rather it is somebody's opinion. Thus, I prefer continuity of past tense present in the first part of the sentence. So, D is the answer.
A - wrong because of 'were'
B - grammatically wrong
E - wrong usage. Should have been 'for them to be successful musicians'
6. The director of the conservatory emphasized to the new students that to become successful musicians not only talent but also hard work were necessary.
(A) to become successful musicians not only talent but also hard work were necessary
(B) in order to be successful musicians requires hard work as well as talent
(C) becoming a successful musician requires hard work as well as talent (D) not only talent but also hard work was necessary to become a successful musician
(E) both hard work and talent were necessary for them becoming successful musicians
Thnx & Rgds,