In reef-building corals, a symbiotic relationship exists between corals and a type of algae that live in their tissues. The corals gain energy and oxygen from this association, while the algae obtain carbon dioxide, nitrogen, and a stable habitat. This type of relationship is best categorized as

Mutualism

Explanation:

In mutualism, both of the organisms benefit from the relationship.

This situation is an example of mutualism, as both the algae and corals benefit.

The algae benefit by receiving carbon dioxide, nitrogen, and a stable habitat. Likewise, the corals also benefit by gaining energy and oxygen.

So, this relationship is best categorized as mutualism.