The correct order of electron affinities is

We know that electron affinity is the amount of energy released when a gaseous  isolated atom accepts electron in its outermost shell.

If an atom makes its electronic configuration stable it means it has higher electron affinity

So, From the  given option   $\mathrm{Cl}>\mathrm{Si}>\mathrm{Na}>\mathrm{Ar}$.

Hence, option(a) is correct.