The correct answer is - Nile.
The letter A on this image shows the river valley of Nile, the letter B shows the Fertile Crescent or the valleys of Tigris and Euphrates, and the letter C shows the valley of the Indus River.
The Nile River valley is one of the river valleys where the first civilizations developed. This river valley provided the right conditions for the people to engage into agriculture, settle in permanently and develop a high sophisticated civilization. The civilization that existed in this part of the world in ancient times is one of the best known civilization, the Ancient Egyptian civilization, known for its colossal buildings like the pyramids and the sphinx, the pharaoh tombs, and the hieroglyphs.