The correct option is B vm>vd
We know that the speed of sound in air is given by,
v=√γPρ=√γRTM
At the same temperature and pressure of air,
v∝1√M .....(1)
Dry air mostly contains nitrogen (78%) and oxygen (≃21%) and other gases like CO2 in traces.
Moist air will contain a higher volume fraction of moisture. The moisture will replace some of the volume occupied by gases like CO2, N2, O2, etc.
Molar mass of H2O is 18 g/mol, which is lesser than the constituent components of dry air.
This will lead to reduction in the effective molar mass of the moist airMmoist air<Mdry air.
⇒Md>Mm .....(2)
From equation (1) and (2) we get,
vm>vd