The correct option is A 40 gram/mole
Argon is a monoatomic gas (n=3)
its molar specific heat at constant volume will be,
Cv=n2R=3R2
⇒ Cv=32×2=3 cal / mol.K
Relation between molar specific heat and specific heat capacity is,
Cv=MwC
∵ (C=0.075 k cal/kg.K, Mw=atomic weight ),
specific heat is always defined as per unit mass
⇒ 3 cal/mol.k =Mw×0.075 cal/g.k
⇒ Mw=30.075=40 gram/mole