Which of Darwin's Ideas Had the Strongest Connection to Darwin

Multiple Choice

Which of Darwin's ideas had the strongest connection to Darwin having read Malthus's essay on human population growth?

A) Descent with modification
B) Variation among individuals in a population
C) Struggle for existence
D) The ability of related species to be conceptualized in "tree thinking"
E) That the ancestors of the Galapagos finches had come from the South American mainland