"in which" basically means "because."

Answers B and C use "in which" and this doesn't really make much sense in the content of the sentence.

ozone is a harmful pollutant, but in the stratosphere because it shields the earth from harmful radiation...? :roll:

Answer E changes the intent of the sentence. Original intent is that ozone has different functionality on the ground and in space. Answer E suggests that ozone is in space.

Answer D uses the word "where" and this changes the dependent clause into phrase. There needs to be additional information to resolve the phrase if used in this format.

but in the stratosphere where it shields the Earth from the most biologically harmful radiation emitted by the Sun

answer needs to be A