Let's look at the answers:
A. Encima de la mesa. - on the table, possible!
BDebajo de la papelera. - under the trash bin - not likely
CEncima de la puerta. - on (top) of the door
DAquí en el reloj. -here in the watch.
A computer is big, so it would not fit into the clock and it would fall from being on top of the door. So the best answer is A - on top of the table!
