Answer
Please see the work below.
Work Step by Step
We know that
a. A reactive, pale yellow gas the atom has a large negative electron affinity so the answer is Fluorine denoted by $F_2$.
b. A soft metal that reacts with water to produce hydrogen so the answer is barium denoted by Ba.
c. A metal which forms an oxide of formula $R_2O_3$ is gallium denoted by Ga.
d. A colorless gas that the atom has a moderately large negative electron affinity is oxygen $O_2$.
