Fifty cents is a plural noun since it is talking about more than one cent. In the example, you would say, "fifty cents <em>is </em>how much I owe you".
This question is asking for the major idea or message that the poem is trying to get across.
A counterargument is just someone disagreeing with the original argument so they come at them with a counter argument so i would say A. To respectfully address any doubts or objections to the claim.
It is kind of like the opposite of your claim, kind of like a counterclaim. Maybe what someone would argue with you claim with. If I said "Cookies are the best dessert because they are sweet", that would be the claim. The rebuttal would be "No it is not, cake is the best because it has frosting".
