According to the diagram, the block should be moving in which direction? a. there is no way to tell which way the block should move. b. the block should move to the right. c. the block should not move at all. d. the block should move to the left.

D. The block should move to the left.

Since the forces are in opposite directions the difference of these forces determines the magnitude and direction of the block.  Therefore,

20 N left - 10 N right = 10 N left

the answer is to check all of them! they all apply well at least i think so!

probably not if net force is zero