You can tell it's sloped because the line of the triangular part of the object that creates the slope is NOT perpendicular to the horizontal base of the object. Also, the proportions for the part of the object jutting out in the figure given and in answer C seem to be off.
This difficult question, but very representative of how some of the questions on keyholes in the DAT will be. Try to practice noticing those proportion differences because some of the answers are pretty close on the real thing!
Good luck!