Atomic radii increases, as the number of shells increases. Thus, on moving down a group atomic radii increases.
The electronic configuration of the given element is
$$
\begin{array}{l}
\mathrm{Mg}_{12}=[\mathrm{Ne}] 3 s^{2} \\
\mathrm{Ca}_{20}=[\mathrm{Ar}] 4 s^{2} \\
\mathrm{p}_{15}=[\mathrm{Ne}] 3 s^{2} 3 p^{3} \\
\mathrm{Cl}_{17}=[\mathrm{Ne}] 3 s^{2} 3 p^{5}
\end{array}
$$
On the other hand, on increasing the number of electron in the same shell, the atomic radii decreases because effective nuclear charge is increases. In $\mathrm{Mg}, \mathrm{P}$ and $\mathrm{C} 1$, the number of electrons are increasing in the same shell, thus the order of their atomic radii is $\mathrm{C} 1 < \mathrm{P} < \mathrm{Mg}$
In case of Ca, the electron is entering in higher shell. So, its atomic radii is highest. Thus, the order of radii is
$$
\mathrm{Cl} < \mathrm{P} < \mathrm{Mg} < \mathrm{Ca}
$$