The rotation of the earth on its axis causes day and night.
The earth rotates on its axis, which is an imaginary line passing through its center, once every 24 hours. As the earth rotates, different parts of it face towards or away from the sun, which causes day and night to occur.
The side of the earth facing towards the sun experiences daylight, while the side facing away from the sun experiences darkness, creating night. The earth's rotation also affects the apparent movement of the sun and stars across the sky.
The varying lengths of day and night and the change of seasons are both caused by other factors, such as the tilt of the earth's axis and its orbit around the sun. Delimiting the tropics refers to defining the geographical boundaries of the tropics, which are regions of the earth surrounding the equator that experience warm temperatures and high humidity.
Therefore, the best fit for the given question is that the rotation of the earth on its axis causes day and night to occur.