Answer:
b. not a valid agreement because the buyer's counteroffer was a rejection of the seller's offer, and once rejected, an offer cannot be accepted later.
Explanation:
Since in the question it is mentioned that the vacant lot is offered for $42,000 but the buyer counteroffers to buy $40,500 but afterward he agreed to the original price of $42,000
So this situation represents that it is not a valid agreement as the counteroffer made by the buyer was rejected by the seller and once it is rejected then the same could not be accepted
Therefore the correct option is b.