The correct answer is option 3 - Stratosphere.

The ozone layer is mainly found in the stratosphere, which is the second layer of the Earth`s atmosphere. The stratosphere is located above the troposphere, which is the layer where weather occurs.

Ozone gas is formed when oxygen molecules are split by ultraviolet (UV) radiation from the Sun. Ozone acts as a protective shield by absorbing most of the Sun`s harmful UV radiation before it reaches the Earth`s surface. This is important because excessive exposure to UV radiation can cause health issues such as skin cancer and eye damage.

The stratosphere is rich in ozone gas due to a natural balance between the creation and destruction of ozone molecules. The high concentration of ozone in this layer helps to maintain the delicate balance of the Earth`s atmosphere and protect life on the planet.

It`s important to note that ozone is not present in significant amounts in the other options listed. The mesosphere (option 1) is the third layer of the atmosphere and does not have a high concentration of ozone. Similarly, the troposphere (option 2) and ionosphere (option 4) do not contain significant levels of ozone gas.

Therefore, the correct answer is option 3 -