The correct option is A 3R
Molar specific heat at constant volume for a gas is,
Cv=fR2
f is the degree of freedom(D.O.F) of gas.
For a triatomic gas( neglecting vibration):
Translational D.O.F=3
Rotational D.O.F=3, since gas mlecules will rotate along x, y, z axis respectively.
⇒f=3+3=6
Therefore,
Cv=6R2=3R
∴Option (a) is correct.