Answer:
Mutualism
Step-by-step explanation:
This is a symbiotic relationship between the algae and the sloth. The algae provides cover for the sloth, and the sloth allows the algae to photosynthesize easier.
Commensalism is a one-sided benefit, which would occur if the sloth helped the algae and received nothing in return.
Parasitism is a harmful form of commensalism, which would occur if the sloth took the algae and used it as cover for its body but killed it in the process.
Competition would occur if they consumed the same of something, which they don't. Instead they are benefitting each other.
And predation isn't occuring because the sloth doesn't eat the algae and the algae doesn't eat the sloth hahahah