You will have to catch the morning flight, so you . . . . . . . . better get ready.

• The word “would” is used to express a possibility or a wish. In this case, the speaker is not expressing a possibility or a wish, but rather a recommendation.
• The word “may” is used to express permission or possibility. In this case, the speaker is not giving permission or expressing a possibility, but rather a recommendation.
• The word “had” is used to express obligation or necessity. In this case, the speaker is not expressing an obligation or necessity, but rather a recommendation.

Therefore, the correct answer is C. should.