Physics~~~ FUN~FUN~FUN~~~

First of all, the wavelength of a person's voice is fixed at a value as a result of his/her fixed length of vocal cord.
So λ = constant.
With reference to the formula of the sound speed in different gases:


圖片參考：http://upload.wikimedia.org/math/f/0/a/f0aa0a8f1de4bb4cd5c26170954f39b3.png

where

R (8.315410 J·mol-1·K-1) is the molar gas constant.
k is the Boltzmann constant
γ (gamma) is the adiabatic index (sometimes assumed 7/5 = 1.4000 exactly for diatomic molecules from kinetic theory, but actually experimentally measured over a range from 1.3991 to 1.403, and 5/3 = 1.6667 for monoatomic molecules).
T is the absolute temperature in kelvins.
M is the molar mass in kilograms per mole.
m is the mass of a single molecule in kilograms.
And also, helium has a higher value of γ than air under the same temperature and pressure, we can say that:
Sound velocity in helium gas > air
Then with the fixed λ, and also the formula v = fλ, we can see that:
Sound frequency in helium gas > air
So the pitch is higher in helium gas and therefore answer = A