Ozone is an allotrope of ______.

oxygen

The correct answer is option 2. oxygen.

Ozone is an allotrope of oxygen.

Allotropes are different forms of the same element existing in different structures.

In the case of oxygen, it has two main allotropes:

Dioxygen (O₂): This is the most common form of oxygen, consisting of two oxygen atoms bonded together. It makes up about 21% of the Earth's atmosphere.

Ozone (O₃): This less common allotrope has three oxygen atoms bonded together. It is present in the upper atmosphere and absorbs harmful ultraviolet (UV) radiation from the sun.

Therefore, ozone specifically arises as an allotrope of oxygen, while the other options (carbon dioxide, hydrogen, and nitrogen) are distinct elements with their own unique properties and do not have allotropic forms.