The correct answer is option 3: Khartoum.

Khartoum is the capital city of Sudan and it is located at the confluence of two major rivers, the Blue Nile and the White Nile. The Blue Nile and White Nile are two tributaries that join together to form the Nile River. The Blue Nile originates from Lake Tana in Ethiopia while the White Nile starts from Lake Victoria in Uganda.

When these two rivers meet in Khartoum, they merge their waters and continue flowing northwards as the Nile River. This confluence is significant as it marks the starting point of the Nile, one of the longest rivers in the world.

Option 1, Port Said, is a city in Egypt located at the northern entrance of the Suez Canal, and it is not associated with the confluence of the Blue Nile and the White Nile. Option 2, Cairo, is the capital city of Egypt and it is situated along the banks of the Nile, but it is not where the Blue Nile and the White Nile meet. Option 4, Alexandria, is a major city in Egypt located on the Mediterranean coast, and it is also not the location of the confluence of the two rivers.