Answer
C. 5'-ATGC-3' with 5'-GCAT-3'
Work Step by Step
The thing to do is to see if the 3' end of one of the putative matches is the correct nucleotide to bind to the 5' end of the other putative match, and then move down the sequence, checking base pair-by-base pair. Only C meets this test, and since A and B are wrong, so is D.