The reason for choosing "should" to fill in the blank is based on the context of the sentence, which involves advice or a recommendation given by the doctor. 

- The sentence starts with "The doctor advised," which implies that the doctor is providing guidance or suggestions.

- "Should" is often used to indicate advice, recommendations, or obligations. In this case, the doctor is advising the person to follow the diet plan.

- "Can" is used to express the ability or possibility of doing something, but it doesn't convey the sense of advice or recommendation needed in this context.

- "Might" suggests a possibility, and "will" indicates future action, but neither of these options is suitable when the sentence is about a doctor's recommendation.

Therefore, "should" is the appropriate choice to complete the sentence because it conveys the idea that the doctor is advising or recommending that the person strictly follow the diet plan for their health.