The answer is b. Darwin's Theory of Natural Selection replaced ideas at the time that didn't adequately offer a mechanism for how evolution could occur over time. The theory of use and disuse, which was put forth by Jean Baptiste Lamarck was an idea that was disproved by August Weismann. Darwin's theory however, explained that living things over-reproduce. He stated that members of a species have variations. There would be a struggle for existence amongst members in a population. Those with the best adaptations might survive, reproduce and pass down those adaptations. Over a long period of time, as these variations accumulated, eventually, evolution of a new species would occur. Darwin also noted that living things had "ancestors" that although looked similar to modern species, had differences as well. He based this on fossils he found on his famous journey to South America and the Galapagos Islands. This too, was a new idea as people in his time period believed all life on Earth was essentially unchanged from when it was first created.
definitely b, ive been studying this in science this year. It is Darwin's theory of natural selection. He believed the species had mutations; the ones that recieved good and beneficial mutations survived and the ones that recieved bad mutations died out. This replaced Jean Lamarck's theory that organisms only changed beneficially. Darwin came up with his theory after he went to the Galapagos Islands and saw the different types of finch (type of bird) on different islands. They had changed due to geography and climate. The ones that were still alive had received good mutations, but the ones that had died out had received bad mutations.