If you live in earth's northern hemisphere, you expect the length of days to change from short in the winter to longest around the summer solstice, june 21st. this is mainly because a) the earth's axis is tilted at about 23.5o. b) of the apparent counterclockwise movement of the earth. c) of the elliptical orbit the earth takes around the sun. d) the tilt of the earth on its axis changes during the year.

The correct answer is "A) the Earth's axis is tilted at about 23.5o". When the Earth completes one revolution on its axis, we have one day; and when it completes one orbit around the sun, we have one year. Besides that, instead of being perpendicular to the sun, the axis of rotation is tilted permanently in 23.5 degrees angle.  This difference in the axis rotation angle causes the northern hemisphere to be exposed to the sun longer, increasing the duration of the day during the summer solstice.