Adverbs modify (describe) verbs or other adverbs and often end in -ly. You need to locate the verb to determine at least one of the adverbs (the action of the sentence.) This is a complex sentence because it has an independent clause (it was not exactly what he had in mind) and a subordinate clause (when he agreed to do the task). If you just look at the independent clause, or the one that can stand by itself, you will find the verb 'was'. Ask yourself 'was what'. The answer is not, which means it is the adverb. To find the adverb describing it, we again ask, 'not what'. The answer is exactly. So the answer is A.
