A policy of collecting colonies and building empires is known as

A. Agrarianisn

B. Industrialism

C. Anti-colonialsism

D. Imperialism

D. Imperialism would be the answer.

In some contexts, Imperialism could be the same as Colonialism. However Colonialism should be used to describe a political control of a more powerful nation over a less powerful one, when it envolves territoty annexation and loss of sovereignty from the annexed nation.

Imperialism is a more general form of control and influence of one nation over another. It may be formal or informal, direct or indirect, of either political or economic nature.