Variation in Daytime and Nighttime

The variation in day and night length is due to the Earth's tilted axis (23.5° inclination) combined with its revolution around the Sun. Option (a) is incorrect - rotation on axis causes day-night cycle but not seasonal variation. Option (b) is incorrect - elliptical orbit has minimal effect on day length; it's the tilt that matters. Option (c) is partially related but not the direct cause. Option (d) is correct because the tilted axis (23.5°) relative to the orbital plane means different latitudes receive varying amounts of sunlight throughout the year. During northern summer, the northern hemisphere is tilted toward the sun, giving longer days, while during winter, it's tilted away, giving shorter days.