From the stated options above, the sentence wherein the verb agrees with the subject is:
A.Somehow, three pennies were blocking the drain.
In this sentence, the three pennies are considered as a number of items therefore must take a plural verb. The other two options are wrong because in the second option, the indefinite pronoun "many" with the plural form of the noun modified must take plural verb "have" and the third option "three dollars" is a value of money so it must take singular verb "is".
