In a Population of Whales,there Is Considerable Variation in the Height

In a population of whales,there is considerable variation in the height of the dorsal fin.Over the course of time,many whales develop very long dorsal fins and many develop very short dorsal fins,but a few whales exhibit an intermediate height in their dorsal fin.What is the most likely conclusion about the variation in size of dorsal fins? 

A)  The population is undergoing directional natural selection. 
B)  The population is undergoing disruptive natural selection. 
C)  The population is undergoing discontinuous natural selection. 
D)  The population is undergoing diversifying natural selection.