Answer:
B. Reaction Time.
Explanation:
In this context, reaction time is what the athlete needs to improve, becomes he has to reduce the time of his response during a spike, in other words, he must respond to the stimulus faster, and that is reaction time.
On the other hand, balance and power aren't correct choices because by improving these, the athlete won't respond faster when someone spikes. Agility doesn't improve that either, because agility is to move easily, not necessarily faster, and it doesn't involve a external stimulus to respond to it.
