The correct answer is option 2: They are all gases at room temperature.

Let`s analyze each option to understand why option 2 is the correct answer.

Option 1: They are all diatomic.

This statement is not true for carbon dioxide, which consists of one carbon atom and two oxygen atoms. Therefore, option 1 is incorrect.

Option 2: They are all gases at room temperature.

This statement is true for hydrogen, nitrogen, oxygen, and carbon dioxide. Hydrogen, nitrogen, and oxygen are gases at room temperature due to their low boiling points. Carbon dioxide is also a gas at room temperature because it exists in a gaseous state under normal atmospheric conditions. Therefore, option 2 is the correct answer.

Option 3: They are all colored.

This statement is not true for hydrogen, nitrogen, oxygen, and carbon dioxide. None of these substances have a visible color. Therefore, option 3 is incorrect.

Option 4: They all have the same reactivity.

This statement is not true for hydrogen, nitrogen, oxygen, and carbon dioxide. These substances have different reactivities. For example, hydrogen is highly reactive, nitrogen is relatively inert, oxygen readily combines with other elements, and