What happens when frequency of spring wave increases?

The correct answer to this question is C. When the frequency of a spring wave increases, the higher frequency shortens the wavelength, and therefore the length decreases. The velocity refers to the speed of the wave, and the frequency does not affect this.

The frequency refers to the number of waves within a certain time period, so therefore as the frequency increases, more waves are required to fit in the same time period, and the wavelength needs to decrease as a result. This can be thought of visually as a horizontal line with many peaks and valleys being drawn within a specific frame representing time.