The correct option is C Predation
The type of population interaction seen between zoochlorellae and hydra is that of predation. Zoochlorellae takes up the carbon die oxide and nitrogen and phosphorous waste of hydra, and in turn, provide them with nutrients and oxygen. Hydra then feeds on zoochlorellae. So, it can be said that hydra feeds upon zoochlorellae. And the relation between them is predation.