Either the boys or Ali __________ responsible for the mistake.

The correct answer is C: is. This question tests your understanding of subject-verb agreement, specifically with correlative conjunctions like "either...or". When "either...or" connects two subjects, the verb must agree with the subject that is closer to it. In this sentence, the two subjects are "the boys" (plural) and "Ali" (singular). "Ali" is closer to the blank space where the verb should be placed. Since "Ali" is a singular subject, the verb must also be singular.

• "Are" and "were" are plural verbs and would be incorrect because "Ali" is singular.
• "Have" is also a plural verb and does not fit the context of being responsible for a mistake in the present tense.

Therefore, "is" is the appropriate singular verb to agree with "Ali". This rule ensures grammatical consistency when dealing with compound subjects joined by "either...or" or "neither...nor", where proximity dictates the verb's form.