D is the answer
It’s cell,tissue,organ, organ system
The coral provides the algae with a protected safe environment (a home) and compounds they need for photosynthesis. The algae in return, produce oxygen and help the coral to remove wastes. Most importantly, zooxanthellae supply the coral with glucose, glycerol, and amino acids, which are the products of photosynthesis.
A cells relatively constant internal physical and chemical conditions are called Homeostasis.
The answer is lithosphere.