Answer:
Option C (more)
Explanation:
Option B and D won't be used because little or less junk food won't make you fat now we are left with option A and option C. Many is used to express quantities that are countable and the word more is used to express quantities that are uncountable and since junk food is uncountable we will be using the word 'more' for this sentence.
Eat more junk food, it makes you fat.
