Earth is known as a blue planet because of which factor?

Over 71 percent of the Earth is covered with water. Therefore from outer space it appears blue and so Earth is called the 'Blue Planet'.

Over 78 percent of the Earth's atmosphere consists of nitrogen which is blue in color. Therefore from outer space it appears blue and so Earth is called the 'Blue Planet'.

Both 1 and 2

Earth appears blue because of the polar ice caps.