Explanation:
The sentence here explains about a disease condition – ulcer. Since ulcer is caused due to increased acid production in the stomach, foods which would increase the acid production must be avoided. The sentence “According to research, foods which increase the stimulation of acid must be avoided...” gives us the hint that if such foods are taken, it will augment the disease condition. The words which suit this context are “exacerbate” and “aggravate”.
The answer choice “decrease” gives an antagonistic meaning to the sentence. The other words given are irrelevant to the given idea and can be eliminated.
When we use it in the sentence, it reads, “According to the research, foods which increase the stimulation of acid must be avoided during ulcer since it may exacerbate/aggravate symptoms like heart burn and stomach pain”.