Points A, B, and C lie along a line from left to right, respectively. Point B is at a lower electric potential than point A. Point C is at a lower electric potential than point B. What would best describes the subsequent motion, if any, of a positively-charged particle released from rest at point B?

Please see below as the answer is self-explanatory.

Explanation:

If the potential at B is lower than A, and the potential at C is lower than B, this means that there is an electric field, directed from A to C.If a positively-charged particle is released at rest at point B, it will be accelerated by the electric field  (which is a force per unit charge, so it produces an acceleration) in the same direction than the field (because it is a positive charge) towards point C.