The correct options are
A It occurs on a deforested site.
D It occurs after a flood or a landslide.
Ecological succession is of two types primary and secondary. Primary succession is a natural process which takes place on barren lands and secondary succession is another type where series of community changes takes place on a previously colonized, but disturbed or damaged habitat. Examples include areas which have been cleared of existing vegetation such as after tree-felling or deforestation and destructive events such as fires, landslides, repeated floods etc.