Oral rehydration salts are used to
a. purify air that has been polluted
b. replenish moisture in the environment after natural disasters such as earthquakes
c. purify water supplies to minimize the risk of catching cholera
d. replenish fluids in the body after it suffers fluid loss from diseases such as cholera

Explanation:

Oral rehydration salts is part of the Oral Rehydration Therapy (ORT), which is fluid replacement to prevent and treat dehydration caused disease like diarrhoea or cholera, which are bacterial diseases that caused extreme dehydration, specially cholera, it can be mortal if it's not rapidly treated.

Basically, this therapy requires to drink a great amount of water with sugar, salt, sodium and potassium content to replace important minerals and ensure a good function of the body.

Actually, these treatments have huge impact, according to the Centers of Disease Control and Prevention, this therapy decreases the risk of death in the 90% of cases, approximately.

Therefore, the correct answer is D, because it's about replacing fluids when having diseases like cholera, which involves diarrhea.