The correct answer is option 3, stratosphere. The ozone gas is primarily found in the stratosphere.

The stratosphere is the second layer of the Earth`s atmosphere, located between the troposphere (the lowest layer) and the mesosphere (the third layer). It extends from about 10 kilometers to 50 kilometers above the Earth`s surface.

Ozone is a molecule made up of three oxygen atoms (O3). It forms a layer in the stratosphere called the ozone layer, which plays a crucial role in protecting life on Earth. The ozone layer acts as a shield, absorbing most of the sun`s harmful ultraviolet (UV) radiation before it reaches the Earth`s surface.

While some ozone is present in other atmospheric layers, the majority (about 90%) is concentrated in the stratosphere. The ionosphere (option 1) is an upper atmospheric layer that contains a small amount of ozone but is primarily composed of ionized particles. The troposphere (option 2) is the layer closest to the Earth`s surface, where weather occurs, and contains only a small fraction of the total ozone present in the atmosphere. The mesosphere (option 4) is the third layer of the atmosphere, located above the str