The ozone layer is found at a height of about 20 to 30 kilometers above the surface of the Earth. It lies in a part of the atmosphere called the stratosphere. This layer is very important for life on Earth.

The ozone layer acts like a shield. It protects all living things by blocking most of the harmful ultraviolet (UV) rays from the Sun. Without this layer, too much UV light would reach Earth, causing more skin cancer, eye problems, and damage to plants and animals.

Though the ozone layer is made up of a gas called ozone (O₃), it is not very thick. In fact, if you brought all the ozone in the atmosphere down to Earth’s surface, it would form a layer only a few millimeters thick. But even this thin layer plays a huge role in keeping life safe.

Let’s review the options:
- 10 to 20 km is below the main range of the ozone layer.
- 30 to 40 km and 40 to 50 km are too high for the densest part of the ozone layer.
- 20 to 30 km is the correct range where the majority of ozone is found.

Due to human activities, some harmful gases (like CFCs) damaged the ozone layer in the past, creating a hole. But thanks to global efforts and international agreements, this damage is slowly being repaired.

So, the right answer is: 20 to 30 kilometers.