Answer:
Amplitude
Explanation:
Basically, when we talk about sound waves, we have certain properties to analyse, where each of them is responsible for a specific effect.
Specifically, in this case, louder sound belongs to bigger amplitude, because the amplitude can be defined as how far is a string from its equilibrium point. In case of a tuning fork, the bigger amplitudes represent a higher movement when it vibrates, which gives louder sounds.
Therefore, the right answer is amplitude.