According to Darwin, evolution occurs

a
during half-life periods of 5,730 years.
b
by chance.
c
very rapidly.
d
due to natural selection.

The answer is d.                                                              

Explanation:

According to Darwin, evolution occurs due to the reason of natural selection.  Because of the characteristics of species are changing from time to time. The characteristics of organisms are also changing frequently. Darwin thought that the color of species is changing day by day. Apart from the darwin thought, practically the body and color of the persons were changing from time to time.  So it is due to natural selection. Therefore due to species evolution is changing naturally. So the answer is d.