Explanation

(b)
In thin layer chromatography the relative adsorption of each component of the mixture is expressed in terms of retention factor $(R_{f})$
$R_{f} = \frac{\begin{array}{r}
distance\ moved\ by\ the\ spot\ centre\ from\ \\
base\ line
\end{array}}{\begin{array}{r}
distance\ moved\ by\ the\ solvent\ from\ \\
the\ base\ line
\end{array}}$