The correct option is A Both assertion and reason are correct and reason is the correct explanation of assertion.
A transverse mechanical wave can travel only in solids but not in fluids, because fluids can't sustain shear stress.
⇒ Assertion is correct.
Air has only bulk modulus of elasticity. However for a transverse wave to travel in the medium, medium should posses shear modulus. It is because transverse wave produces shear stress in the material medium through which they propagate.
⇒ Reason is also correct and it is the correct explanation of assertion.
Hence, option (a) is correct.