The correct option is A Principle of geology
Darwin got the idea of the population from Malthus and got the idea of the principle of geology from Lyell. Charles Darwin read while on the Beagle and was much influenced by, Lyell's Principles of Geology. His own experiences during the voyage backed up the geologist's theories about how the shifting crust of the Earth was one of these long-acting, gradual, planet-sculpting forces. Darwin reckoned that volcanoes were connected by magma underground and that pressure not released through eruptions could cause earthquakes. Thus these different phenomena were different aspects of the forces Lyell believed had operated over geological time to produce the world we see now. Hence option A is correct.