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

$\mathrm{Cl} < \mathrm{P} < \mathrm{Mg} < \mathrm{Ca}$