Question

Twenty dollars _________ not a fair price for that old tape player.

Answer 1

It would be C, "is" if you say any of the other ones with that sentence it sounds silly.

Answer 2

The answer would be C.

Twenty dollars is not a fair price for that old tape player.

It couldn't be are. since are is used for plural nouns. "Tape player" is singular, therefore, it is not are.

Be is a connecting verb. It connects certain words, but in this case, it isn't grammatically correct.

"Were" is the past subjunctive of "be". Were is usually used in past tense and is second person singular past and plural past.

Have a wonderful day! :)