Assertion (A) is true because electromagnetic waves carry energy and momentum. When they strike a surface, they transfer momentum to it, which results in a force and consequently exerts radiation pressure on the surface.
Reason (R) is true because electromagnetic waves consist of photons, which have zero rest mass. Thus, there is no rest mass associated with electromagnetic waves.
However, the pressure exerted by electromagnetic waves is due to the momentum they carry (p=cE), not because they are massless. Therefore, both (A) and (R) are true, but (R) is not the correct explanation of (A).
Answer: Both (A) and (R) are true but (R) is not the correct explanation of (A)