Final answer:
If Earth's axis had no tilt, the overhead sun would always be at the equator, and there would be no variations in seasons as we experience them due to a constant angle of sunlight throughout the year.
Step-by-step explanation:
If Earth's axis had no tilt, the correct answer to what would happen is: c. the overhead sun would always be at the equator. The familiar cycle of the seasons is a result of Earth's 23.5° axis tilt. If the Earth's axis were not tilted, there would be no variation in solar energy at a given latitude throughout the year. Instead, each point on Earth's equator would experience the sun at its zenith every day, and the concept of seasons as we know them, defined by significant changes in daylight length and sunlight intensity, would not exist.
Thus, statements like seasonal changes in solar energy being maximized or solstices and equinoxes being well-defined, or the poles alternating between day-long darkness and day-long sunlight over the course of the year, would all be incorrect as these phenomena are directly linked to the tilt of Earth's axis.