P<Cl<Ca<Mg
Ca<Mg<P<Cl
Mg<Ca<Cl<P
Cl<P<Mg<Ca
The atomic radii decreases on moving from left to right in a period, thus order of sizes for Cl, P, and Mg is Cl<P<Mg. Down the group size increases. Thus overall order is :Cl<P<Mg<Ca. So, option D is correct.