Which of the following would cause gene flow?
a. artificial selection
b. individuals moving into a population
c. genetic drift
d. mutation

Your answer would be B. individuals moving into a population. 

Imagine you live in a small village that is remote and hasn't had much contact with people from different geographical areas or other continents even. Your community will have a certain gene pool as each individual has certain genes and when the genes of every person are taken into account this leaves us with certain genes that perhaps aren't found somewhere else. Suddenly, there is a big influx of another group of people coming from one particular region with a somewhat different gene pool to this community.

After some time, these people will reproduce and create new children who will have a more diverse gene pool (if the people from the old and new community would make babies for example). This is how the gene pools would get expanded and also gene flow would occur.