When the students move the ends of the rope up and down faster while maintaining the same distance of movement, the frequency of the wave increases. This means that the waves are produced more frequently in a given time period.
According to the wave relationship, the speed of the wave (which remains constant if the medium does not change) is equal to the wavelength times the frequency (v = fλ). If the frequency increases and the speed remains constant, the wavelength must decrease.
With the amplitude remaining unchanged (since the distance moved up and down remains the same), the correct answer is:
The frequency would increase and the wavelength would decrease, but the amplitude would be unchanged.