Which of the following was most influential upon Darwin's formulation of theory of natural selection?

Mutation theory of Vries takes sudden, random and discontinuous variation as raw material for evolution while Darwin advocated continuous variations. Darwin and Wallace independently advocated the “survival of fittest” to explain the process of evolution. By the time, Wallace sent his paper entitled “ On the tendency of varieties to depart indefinitely from the original types”, Darwin already has developed his concept of organic evolution. Darwin did not advocate Lamarck’s concept of inner vital force and inheritance of acquired characters. Malthus, in Malthus’s essay on human population, proposed that living organisms multiply in geometric ratio. Darwin applied this notion as genetic potential of organisms and struggle for existence in his theory of natural selection. Thus, the correct answer is option C.