There are two forces applied to the pendulum: gravity and tension. Gravity always point down and tension always points along the string. Therefore, when the bob is at position , the force (and therefore the acceleration) must be vertical (up, down, or ). Therefore, only answers (C) and (E) can be correct. Now, answer (E) cannot be correct for the following reason. The gravity force vector and the tension force vector are antiparallel at position , but in all of the other positions shown these forces are neither parallel nor antiparallel. Therefore, their sum cannot be in the same direction as the tension force vector (along the string). Therefore, answer (C) is correct.
