A

D. It occurs only through geographic forces.

Explanation:

Biogeographic isolation is the separation of two populations of the same species due to physical factors. It may lead to evolution, as the groups will adapt to their new circumstances, which may be dramatically different between the two locales. However, geographic isolation cannot be described as one of the evolutionary mechanisms, as they are generally biological in nature.

Geographic Isolation Example Using Foxes.
Geographic Isolation Example Using Foxes (Source: http://slideplayer.com)

Geographic isolation may lead to the emergence of new species but does not necessarily create them. The term means that members of different species are unable to produce fertile offspring.

However, many animals that are slightly different due to geographical factors and isolation, including humans, can breed without any issues. As such, the first three options are invalid, and only the fourth is correct.