Which of these protects the earth from harmful radiation when found in the stratosphere, but is a pollutant at lower altitudes? carbon oxygen ozone hydrogen

The correct answer is - ozone.

The ozone is essentially three molecules of oxygen, or O³. This gas is crucial for the life on the Earth because it is the one that manages to filter out big portion of the UV radiation from the Sun which is extremely harmful if its too strong, as it will contribute to the dying out of pretty much every living organism.

While the ozone layer in the stratosphere is very useful, the ozone in the lower parts of the troposphere is actually considered as pollutant. The reason why it is considered as pollutant is because it is very harmful if inhaled, and it also heavily damages the plants.

fertilization is the union of an egg and a sperm cell i believe.