For this case we can translate the sentence given as:
We have to follow these rules _________ save the environment.
We must complete the sentence with the most appropriate option.
By definition, we use the preposition "para" when we know something with certainty.
Then, according to the sentence given, if certain rules are followed, the environment can be saved.
Thus, the complete sentence is:
We have to follow these rules to save the environment.
Tenemos que seguir estas reglas para salvar el medio ambiente.
Answer:
Option C