The correct answer is A. Much ... effect
Explanation
To ask questions in English you can use "how much" and "how many"; these two differ in that the first is used to ask for uncountable nouns such as air, salt, sugar, among others, while the second is used for countable such as apples, people, animals, hours, among others. Therefore, the most appropriate word to ask about the amount of salt that is healthy to eat per day is "how much". On the other hand, the word "affect" is a verb that refers to action, while "effect" is a noun that refers to the consequences of an event. So, the most appropriate is the noun because the text is wondering about the effect that salt has on health and not how it "affects" health. So the answer is A. Much ... effect.
