Choice "C" is correct. A contract generally must be supported by valid consideration. Valid consideration will be present if there is a bargained for exchange of something of legal value. If the act promised has already been performed, the bargain element fails. Thus, it is said that past consideration is no consideration.
Choice "d" is incorrect. A contract generally must be supported by valid consideration. Relinquishment of a legal right constitutes something of legal value. Thus, this is not a good defense for Grove.
Choice "b" is incorrect. As long as the consideration is not a sham, the courts will not inquire into the adequacy of the consideration exchanged. $400 is not sham consideration; thus, the large disparity in value of the consideration exchanged here is not a defense.
Choice "a" is incorrect. The benefits of a contract need not flow to the parties to constitute consideration, the mere giving of a benefit or receipt of a detriment is sufficient.