Option 1: This option states that ozone is found in the Earth`s atmosphere mostly between 5 and 10 km. This is incorrect. Ozone is not typically found at this altitude range.

Option 2: This option states that ozone is found in the Earth`s atmosphere mostly between 10 and 20 km. This is incorrect. Ozone is not typically found at this altitude range either.

Option 3: This option states that ozone is found in the Earth`s atmosphere mostly between 20 and 50 km. This is the correct answer. The ozone layer is primarily located in the stratosphere, which is between approximately 20 and 50 kilometers above the Earth`s surface. This layer contains a high concentration of ozone molecules that play a crucial role in absorbing and filtering out the majority of the Sun`s harmful ultraviolet (UV) radiation.

Option 4: This option states that ozone is found in the Earth`s atmosphere mostly between 50 and 100 km. This is incorrect. The ozone layer is not typically found at such high altitudes.

Therefore, the correct answer is option 3, which states that ozone is found in the Earth`s atmosphere mostly between 20 and 50 km.