Answer: Option C
Step-by-step explanation:
The coral reef has its own biodiversity and has many types of animals and plants living in there. The coral reef contains plankton which trap sunlight and convert the solar energy into chemical energy which is used by other animals of the biodiversity.
The source of sunlight is the main source of energy and if there will be more sea weed trapping sunlight then there will be a competition between seaweed and coral reef.
So, the natural reason for the decreased biodiversity is overgrowth of population of seaweed which prevents the light from reaching the reef.