Explanation:
The correct answer is option B: Earth is inclined towards its orbital plane.

Revolution of the Earth:
The revolution of the Earth around the Sun is responsible for the change in seasons. It takes approximately 365.25 days for the Earth to complete one revolution around the Sun. However, the revolution of the Earth does not directly cause the phenomenon of six months long day or night in the polar regions.

Rotation of the Earth:
The rotation of the Earth on its axis causes day and night. As the Earth rotates, different parts of the planet are exposed to sunlight, resulting in the alternation between day and night. However, the rotation of the Earth does not explain the six months long day or night in the polar regions.

Inclination of the Earth:
The correct explanation for the six months long day and night in the polar regions is the inclination of the Earth towards its orbital plane. The Earth's axis is tilted at an angle of approximately 23.5 degrees with respect to its orbital plane. This tilt causes different parts of the Earth to receive varying amounts of sunlight throughout the year.

Effect on Polar Regions:
During one half of the year, the North Pole is tilted towards the Sun, while the South Pole is tilted away from the Sun. This results in the Northern Polar region experiencing six months of continuous daylight, known as the "Midnight Sun," while the Southern Polar region experiences six months of continuous darkness, known as the "Polar Night."

Changes in Daylight:
As the Earth continues its revolution around the Sun, the tilt of the Earth's axis causes the amount of daylight received by the polar regions to gradually change. This leads to the transition from six months of continuous daylight to six months of continuous darkness in the North and vice versa in the South.

In conclusion, the reason for six months long day in the Northern Polar region and six months long night in the Southern Polar region is the inclination of the Earth towards its orbital plane. This tilt results in varying amounts of sunlight received by different parts of the Earth throughout the year.