What was the name given to the belief that life is made up of competitive struggles and only the fittest survive? a. social darwinism b. mercantilism c. militarism d. survivalist order

A.  Social Darwinism

A quote I found sums this question up, but it'd be better to do some side research. 
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"the theory that individuals, groups, and peoples are subject to the same Darwinian laws of natural selection as plants and animals. Now largely discredited, social Darwinism was advocated by Herbert Spencer and others in the late 19th and early 20th centuries and was used to justify political conservatism, imperialism, and racism and to discourage intervention and reform."