The sky is clear, it . . . . . . . . rain today. A. may B. might C. could D. should

The correct answer is A. may.

The other options are incorrect because they do not express the same level of uncertainty as may.

• Might is used to express a possibility that is less likely than may.
• Could is used to express a possibility that is even less likely than might.
• Should is used to express a suggestion or recommendation.

In this case, the sky is clear, so it is not very likely that it will rain today. However, there is still a small possibility that it could rain, so the best answer is may.