The correct answer is "They are used for photosynthesis".
Explanation:
Kelp is a type of seaweed that lives in underwater coastal zones under formations of multiple organisms known as "underwater forests". Autotrophic organisms such as kelp, use carbon dioxide (CO2) and oxygen (O2) to peform its photosynthesis processes. During photosynthesis, the organisms use CO2, water and sunlight to produce glucose, while O2 is also produced but as a byproduct.
