Thin layer chromatography (TLC) is another type of adsorption chromatography, which involve separation of substance of a mixture over a thin layer of an adsorbent coated on glass plate.
A thin layer (about 0.2mm thick) of an adsorbent (silica gel) or (Alumina) in spread over a glass plate of suitable size.
The relative adsorption of each component of the mixture is expressed in terms of its retardation factor.

Hence Assertion (A) is correct and Reason (R) is correct explanation of (A)