Introduction:
In this question, we are given the statement "You have to drink _______ coffee" and we have to choose the correct option. Let's analyze the options and determine the correct answer.

Analysis of Options:
a) little: This option suggests that the quantity of coffee to be drunk is small.
b) less: This option indicates that the quantity of coffee to be drunk is comparatively smaller than something else.
c) Both a and b: This option implies that both options a and b are correct.
d) none of these: This option states that none of the given options is correct.

Explanation:
To determine the correct answer, we need to consider the context of the sentence. The sentence states "You have to drink _______ coffee." Here, the word "have to" implies that drinking coffee is necessary or mandatory.

In this case, the correct option would be "less" (option b) because it suggests that the quantity of coffee to be drunk is comparatively smaller than something else. This implies that the person should limit their coffee intake and consume less coffee than they usually would.

Option a, "little," suggests a small quantity of coffee but it does not convey the idea of moderation or limitation. Therefore, it is not the correct answer.

Option c, "Both a and b," is not the correct answer because option a alone does not convey the desired meaning.

Option d, "none of these," is also not the correct answer because option b, "less," is the appropriate choice in this context.

Conclusion:
Therefore, the correct answer is option b, "less," as it accurately conveys the idea of limiting or reducing coffee intake.