Fill in the blank with the most appropriate word.
An adverb is a word or phrase that qualifies or changes an adjective, verb or another adverb, to express a relation of a situation, a cause etc. Option C) ago is correct as it means a long time back from now which is suitable in the given sentence. The other options are wrong as last, when, and for are not at all appropriate for the sentence given to us. The correct answer is C) ago.