The correct answer is (D): The tilt of the Earth on its axis changes during the year.
Earth revolves around the Sun as it spins on its axis of the rotation once a day. While the Earth orbits the Sun, the tilt of Earth's axis stays lined up with the North Star resulting to the North Pole tilting towards the Sun. The Northern Hemisphere is normally tilted toward the sun on the summer solstice while the southern hemisphere is tilted toward the sun on the winter solstice. Northern Hemisphere hence expects a change in days length where the days change from short days in the winter to long days around the summer solstice on June 21st.