b
With the increase in the number of electron in the same shell, the atomic radii decrease due to increase in effective nuclear charge. However, atomic radii increases, as the number of shells increases. Thus, on moving down a group atomic radi increases.

The electronic configuration of the given elements is

\(\mathrm{Mg}_{12}=[\mathrm{Ne}] 3 \mathrm{s}^{2}\)

\(\mathrm{Ca}_{20}=[\mathrm{Ar}] 4 \mathrm{s}^{2}\)

\(\mathrm{P}_{15}=\left[\mathrm{Nel} 3 \mathrm{s}^{2} 3 \mathrm{p}^{3}\right.\)

\(\mathrm{Cl}_{17}=[\mathrm{Ne}] 3 \mathrm{s}^{2} 3 \mathrm{p}^{5}\)

In \(Mg, P\) and \(Cl\), the number of electrons is increasing in the same shell thus, the order of their atomic radii is

\(\mathrm{Cl} < \mathrm{P} < \mathrm{Mg}\)

In \(Ca\), the electron is entering in the higher shell, thus, it has the highest atomic radii among the given. Thus, the order of radii is

\(\mathrm{Cl} < \mathrm{P} < \mathrm{Mg} < \mathrm{Ca}\)