Wavelength is inversely proportional to the _____________.

Wavelength is the distance between the two successive compressions of a sound wave which is measured in terms of centimetres and millimetres.

The wavelength of a sound is inversely proportional to its frequency and it is given as:
$wavelegth \left(\lambda \right) = \frac{velocity \left(v\right)}{frequency \left(f\right) }$

therefore, 'Frequency' is the correct response.