The possible answers are
a. the formation of a tall sand dune on a previously grassy beach
b. a vast glacier in motion forming a U-shaped valley on its wake
c. a volcanic eruption that covers most of what used to be a forest
d. an introduced species that devastates native plants as it feeds
The correct answer is D
Secondary succession is an ecological process that happens when plants colonize a new habitat.
In primary succession, plants colonize a completely new, barren land with no soil present.
In secondary succession, the colonization happens in a habitat that has already been populated by plants and other forms of life, but the habitat has been damaged in some way that led to the opening for a place in the habitat for some new organisms.
In our example with the introduced species, the devastation of the native plants opens a space in the ecosystem for other forms of life to take their places.