To analyze the situation regarding the nail and the action-reaction pair, it's important to understand Newton's third law of motion, which states that for every action, there is an equal and opposite reaction.
If the only force acting on the nail is downward (such as gravity, or if it's being struck by a hammer), there is an unbalanced force acting on it. In this case, the nail will accelerate downward because the gravitational force acting on the nail will result in a net force. The action force (the force applied to drive the nail into a surface) and the corresponding reaction force (the force exerted by the nail against the action force) do not directly balance out if no opposing force is present to counteract the downward motion.
Therefore, the correct conclusion is:
Accelerate downward because there are unbalanced forces.