The answer is (c): a correct top view consists of a square with two extra lines - an X with its ends at the corners of the square. You can see the top view by passing your mouse over the drawing below.
Do you now see what it is?
The full drawing and the actual machined piece can be seen by clicking your mouse here.
Can you now see the other solution? If you cut the piece into two halves with a single cut across one of the diagonals of the top view, this piece also satisfies all of the requirements given in the question. I guess this illustrates why you usually need three drawings to completely define something to be machined.
