Enemos que seguir estas reglas salvar el medio ambiente. a. para entonces b. por c. para d. por eso

The right option is:

- Tenemos que seguir estas reglas para salvar el medio ambiente.

Explanation:

The translation of the sentence is:

- We have to follow these rules to save the environment.

The word "para" has many uses in Spanish within which we can mention:

It is precisely the latter mentioned that is applied in the sentence, since the purpose of following the rules (as mentioned in the first part of the compound sentence) is to save the environment, so it is appropriate to use the word "para."