Concept:The term “Earth Day” refers to the time Earth takes to complete one full spin on its axis.
Explanation:Earth’s axis is an imaginary line tilted at 66½° to its orbital plane.
Due to the spherical shape, only half of Earth faces the Sun at any moment – that half experiences day, the other half night.
The circle separating day and night is called the circle of illumination.
Earth finishes one rotation about its axis in roughly 24 hours.
This complete rotation period is known as an “Earth Day.”
It is the daily motion of Earth.
Other options are incorrect: sunlight duration (A), Greenwich standard time (B), and revolution around the Sun (D).
Answer:C. Period of rotation of the earth around its axis.