Answer
The speed of sound is faster in helium. The frequency must increase if the speed of sound increases.
Work Step by Step
The speed of sound $v \propto \sqrt{\frac{1}{M}}$, where $M$ is the molar mass of the gas.
Since the molar mass of helium is less than the molar mass of normal air molecules, the speed of sound is faster in helium.
We know that $v = f~\lambda$. Since the wavelength does not change, the frequency must increase if the speed of sound $v$ increases.
