The correct answer is sympatric speciation. Although rare, sympatric speciation occurs in populations that occupy the same geography, but still develop reproductive isolation. This isolation is generally driven by behavioral differences. Allopatric speciation occurs in biogeographically distinct populations. Parapatric speciation occurs when there is a small hybrid zone or overlap of two biogeographically distinct populations. Balancing selection refers to a mode of natural selection, not speciation.

The correct answer is allopatric speciation. Sympatric speciation occurs when two biological populations inhabit the same geography, but speciate due to behavorial differences. Parapatric speciation occurs when there is a small hybrid zone or overlap of two biogeographically distinct populations. Disruptive selection and natural selection do not explain events of speciation in the context of the question.