Select the best word for the blank in the following sentence.
My uncles, who used to work in the limestone quarry, _________ now started their own business.

A subject must agree with its verb in number. The subject in this sentence (My uncles) is plural so we need a plural verb (have).
The trap here is that the subject is separated from the verb. When checking for subject-verb agreement, look at just the subject and the verb. Ignore everything else.
My uncles, who used to work in the limestone quarry, have now started their own business.