It's a). Negative dynamic stability implies a divergent oscillatory motion once perturbed (vibration with increasing amplitude) which brings the vehicle back past it's intended position, and then past it with increasing rates in each period. With true negative static stability the vehicle never returns to its intended position. It can be both so your train of thought is probably correct, but a) is the only one implied in the question and required for the described outcome. b) could also be true at the same time but is optional.