Answer
Please see the work below.
Work Step by Step
a. A reactive nonmetal, the atom has a large negative electron affinity.
So the answer is chlorine ($Cl_2$)
b. A soft metal, the atom has low ionization energy, hence the answer is sodium($Na$)
c. A metalloid that forms an oxide of formula $R_2O_3$, so the answer is $Sb$
d. A chemically unreactive gas ,so the answer is Argon ($Ar$).
