The correct answer is C. both (a) and (b).
The Earth rotates on its axis once every 24 hours, which is what causes day and night. The Earth also revolves around the Sun once every 365.25 days, which is what causes the seasons.
Option A is correct because the Earth rotates on its axis. Option B is correct because the Earth revolves around the Sun. Option C is the correct answer because both rotation and revolution are major motions of the Earth. Option D is incorrect because it is not always the case that the question under construction will require a discussion of both rotation and revolution. For example, a question about the Earth’s atmosphere would not require a discussion of the Earth’s rotation.