The answer to your question is A. - Them because you are referring to the crumbs and just not something random, in which case you would use "it". I hope this helps you! Have a great day!!
The word 'them' is a <em>personal pronoun in the object form for the personal</em> <em>subject pronoun</em> 'they' (third person plural). In the sentence A, the eating of the cookies resulted in crumbs on the floor and the counter, which left them both dirty, so the responsible knew that they had to clean them up. The antecedent of <em>'them'</em> are <em>'floor and counter'=they </em>
