Final answer:
Charles Darwin drew upon geology, paleontology, taxonomy and systematics, demography, and evolutionary biology to form his theory of evolution. So the correcct option is a.
Step-by-step explanation:
The five scientific disciplines that Charles Darwin drew upon to develop his thinking about evolution were geology, paleontology, taxonomy and systematics, demography, and evolutionary biology. Darwin was influenced by the work of geologists like Charles Lyell, whose findings suggested that the Earth was much older than previously believed, allowing ample time for the evolution of life. He also studied fossils, which helped him understand that life forms have changed over time. Darwin's taxonomy and systematics observations made clear that organisms fell into 'natural' groupings, which was a consequence of evolutionary processes. Demographic studies provided insight into population dynamics, variations, and species survival. Lastly, evolutionary biology as a discipline helped integrate these observations into a cohesive theory of evolution by natural selection.