The world's longest river, the Nile, flows through the continent of Africa. It is approximately 6,650 kilometers (4,130 miles) long and is a significant geographical feature in northeastern Africa. Therefore, the correct answer is (B) Africa.

The Constitution of India establishes a federal system of government. It contains all the usual features of a federation Viz., two government, Division of powers, written constitution, Supremacy of constitution, Independent judiciary and Bicameralism. Article 1 says that India is a union of states which implies two things: first, Indian federation is not the result of an agreement by the states and second, No state has the right to secede from the federation.