(A) : Transverse mechanical waves can not propagates in liquids and gases.
(R) : Liquids and gases flow when acted on by shearing stress, they can not sustain shear stress.

The medium needs to be elastic in order to apply a restoring force when shearing stress is applied, which is necessary for the transmission of a transverse mechanical wave. Nevertheless, liquids and gases cannot withstand shear stress to produce restoring force; instead, they begin to flow when shear tension is applied.