Which style of music was most popular during the classical?
a) sacred
b) opera
c) secular
d) vocal

The answer is B) Opera.

Explanation:

The "classic" is that period between the 18th and 19th centuries.

In that period, not just any music was popular with the people, since the aristocracy that replaced the church decided to take the reins as sponsors of the music of the time.

The aristocracy wanted to listen to harmonious and elegant music, this is how the opera of the baroque composition took control of the classical era over all other genres as the most important.