Answer
(a) Sometimes true. (b) Never true. (c) Always true. (d) Sometimes true.
Work Step by Step
(a) The skateboarder could be moving in any direction and have a force acting on him; it doesn't have to be the direction of his motion. (b) Since $\vec{F}=m\vec{a}$, the acceleration is in the same direction as the force. Therefore, acceleration cannot be at a right angle to the net force. (c) Since $\vec{F}=m\vec{a}$, the acceleration is in the same direction as the force. (d) The direction of the force acting on the person doesn't depend on the velocity of the person. He could be at rest, but it doesn't mean that he must be at rest.