Why is it likely that most speciation is allopatric?
a. because disruptive selection between forms of traits in sympatric populations is always very strong.
b. because gene flow between diverging populations is strong in allopatry.
c. because related species are always found in allopatry rather than in sympatry.
d. because vicariance events are common.
e. because gene flow between populations prevents genetic divergence in sympatry.

e. Because gene flow between populations prevents genetic divergence in sympatry.

Explanation:

Speciation requires genetic divergence between populations of a species to isolate them reproductively. however, when present in the same habitat, gene flow between populations of a species does not allow their reproductive isolation and thereby, prevents the formation of new species. These populations need strong episodes of disruptive selection for their reproductive isolation.  

On the other hand, two geographically isolated populations can not interbreed and thus, there is no gene flow between them leading to their genetic divergence and speciation. So, allopatric speciation is more common than sympatric one.