Final answer:
Charles Darwin, not Einstein, Galileo, or Newton, posited that current observations can explain past events, a concept integral to his theory of evolution by natural selection.
Step-by-step explanation:
The scientist who stressed that scientists can explain past events in terms of processes they can observe happening now is c) Darwin. This concept aligns with Darwin's theory of evolution by natural selection, which he observed and supported with evidence from the natural world. The principle that the same natural laws and processes that operate in our present have operated in the past, and at similar rates, is known as uniformitarianism. Darwin's observations and hypothesis suggested that slow, cumulative changes could, over vast stretches of time, lead to the biological diversity we see today.
While Einstein's theory of relativity and Newton's laws of motion are significant in the progression of physics and scientific models, they are not associated with the idea emphasized in the question. Both physicists contributed immensely to our understanding of physical laws, but the question specifically refers to biological processes and the historical understanding of Earth's changes, which ties back to Darwin's work in biology.