Which of the following groups contains elements that are mostly diatomic gases?

The group that predominantly consists of diatomic gases is the halogens. This group includes elements such as fluorine (F2) and chlorine (Cl2), both of which naturally exist as diatomic molecules in their gaseous forms under standard conditions. These diatomic gases are characterized by their strong covalent bonds between two atoms of the same element, which allows them to be stable and exist as gases.

In contrast, the other groups mentioned do not primarily contain diatomic gases. For instance, alkali metals and alkaline earth metals are typically found as solids at room temperature and form metallic or ionic compounds rather than diatomic gases. Noble gases, while they exist as single atoms rather than molecules, are gases but do not form diatomic gases naturally. Therefore, among the groups listed, halogens are distinct for their tendency to exist as diatomic molecules.