These four elements are most likely in group a.15 b.16 c.17 d.18

Option (C) is the correct answer.

Explanation:

At room temperature, only bromine and mercury exist as a liquid. Whereas diatomic molecules and some halogen atoms exist as gases at room temperature.

Gases which exists at room temperature are hydrogen, nitrogen, oxygen, fluorine, and chlorine. On the other hand, most of the elements exist as solid at room temperature like carbon, iodine, sodium etc.

Also, it is known that boiling point of bromine is 332 K, chlorine is 239 K, iodine is 457 and fluorine is 85 K.

Elements F, Cl, Br, I are all halogen atoms and belong to group 17.

Hence, we can conclude that given four elements are most likely in group 17.

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)