• "In the 1970s scientists found out that chemicals_____ into the atmosphere______the ozone layer."

The option with the "released/had been destroying" is indeed grammatically correct, however it cannot be true.

The sentence reads as follows:

• "In the 1970s scientists found out that chemicals that were released into the atmosphere had been destroying the ozone layer."

So because of the chemicals the ozone layer was being under the process of destruction and was destroyed by the time the scientists figured it out.

Still, since this question concerns only grammar the option that says it is correct is indeed correct.

• "In the 1970s scientists found out that chemicals_____ into the atmosphere______the ozone layer."

The option with the "released/had been destroying" is indeed grammatically correct.

The sentence reads as follows:

• "In the 1970s scientists found out that chemicals that were released into the atmosphere had been destroying the ozone layer."

So because of the chemicals the ozone layer was being under the process of destruction and may have been possibly destroyed by the time the scientists figured it out.

Still, since this question concerns only grammar the option that says it is correct is indeed correct.