What Was the Philosophy Behind the Briefly Popular Social Darwinism

What was the philosophy behind the briefly popular Social Darwinism practiced in the late 1800s and early 1900s?

A) Utilitarianism was the best way to approach all problems.
B) Sociology was a pseudo-science when compared to Darwin's theories.
C) Darwin was a social creature and his teachings led to harmony.
D) Only the fittest will survive as a ruthless natural order dictates.
E) Socialists are meant to overcome all resistance in politics.