When temp. increases, then frequency of tuning fork increases or decreases? Explain.

The frequency of vibration of a tuning fork involves Young's modulus, an increase of which results in increased frequency of vibration. So the frequency will change in the same direction as the change in Young's modulus with temperature. Young's modulus characterizes the "stiffness" of a material.
I haven't yet found out what that temperature dependence is, but I suspect that materials, and specifically the metal of a tuning fork, lose some stiffness with increasing temperature, which would cause the tuning fork's frequency to drop slightly with increasing temperature. So my thought is:
decreases
Regards,
Nirmal Singh
Askiitians faculty