Answer: The frequency.
Explanation:
Each time that two consecutive troughs pass through a fixed point, means that the wave did a complete lapse, and this time-lapse is called the period of the wave.
Now, if we increase the number of troughs that pass in a fixed amount of time, this means that the period of the wave decreases (this means that the period is a little smaller)
Now, as we know, the frequency is inversely proportional (T = 1/f where T is the period and f is the frequency) to the period of the wave, so this would mean that the frequency is being increased
