The larger the frequency of the tuning fork,

the smaller the length of the tube at which you hear the resonant sound.
Explain why this is so (think in terms of the relationship between frequency and wavelength).

2 answers

Wavelength = V/F.
V = The speed of light.
F = Frequency.

The wavelength is inversely proportional to the frequency. In other words, if the frequency is doubled; the wavelength is cut in half.
the larger the fork the smaller the frequency