Cause(s) populations to change over time.

natural disasters
natural selection
weather
genes

The answer is B., Natural Selection.

Weather certainly doesn't change populations, and genes only caused individuals in a population to be slightly ( genetically-wise ) different from the others. It does cause change, but not change in the whole population over time.
Natural disasters don't change the populations over time either. Natural disasters only caused change in their environment, at most.

So, the only answer left is natural selection, and it makes sense too! 
Natural selection is the process where the individuals with better traits suited to survive in that specific environment live on and give those good traits to their offspring.
The individuals with less suited traits to survive will die out, and will not be given an opportunity to reproduce and pass on their less suited traits, so over time, the population will increase of individuals with better suited traits to survive and the individuals with less suited traits will eventually die out, therefore making the entire population change.

Hope I helped you!