Final answer:
Dispersal is the movement of a few members of a species to a new geographic area, while vicariance occurs when a natural situation physically divides organisms. Dispersal can lead to limited exchange or none at all between populations, while vicariance results in the separation of once continuously distributed populations.
Step-by-step explanation:
Dispersal and vicariance are two categories of allopatric processes in biology. Dispersal refers to the movement of a few members of a species to a new geographic area, while vicariance occurs when a natural situation physically divides organisms. Dispersal can lead to limited exchange or none at all between populations, while vicariance results in the separation of once continuously distributed populations. Therefore, statement 4) a and c are correct.