Why Have the Two Plant Populations Depicted in the Diagram

Why have the two plant populations depicted in the diagram below become so different in the time they were separated by the ocean?

A) Founder's effect; a small group of individuals carrying only the tall allele established a new population on the right side of the ocean.
B) The amount of gene flow between the two populations increased when the ocean separated the plants, so mutations moved from one population to another.
C) When sea levels rose, it killed all of the plants on the right side of the figure. A new species evolved from another plant species already existing in the mountains on the right.
D) The separated populations adapted to the different environments on each side of the ocean and became reproductively isolated.