Felix and fanny mendelssohn made their concert debuts a. in a tavern b. as the opening act for beethoven c. at home d. in concert halls

The correct answer option is A. In concert halls.

Explanation:

Both Felix Mendellsohn and his sister Fanny Mendelssohn Hensel were the famous music composers in the era of 19th century.

Fanny Mendelssohn was a highly competent musician but her family thought that music was not a respectable career for a young woman. So Fanny made her debut as a soloist in Felix's piano concert at a benefit concert (in hall).

not concrete, existing in thought or as an idea but not having a physical or concrete existence

many is the pronoun that is used in this sentence. many is a quantifier that determines the amount of discoveries. when we use quatifiers followed by nouns, they act as determiners, but when they are used without a noun, as it is this case, they are pronouns.