The correct answer is - mutualistic symbiotic relationship.
The mutualistic symbiotic relationship is a relationship between two or more species, in which they collaborate and have mutual benefit of their collaboration.
In this case, the pseudomyrmex ants and the acacia trees do exactly that, they collaborate for the benefit of both of them. The acacia tress provide shelter, nectar, and protein for the ants, and the ants are protecting the tree from herbivorous insects and parasitic plants. In this way, both of them benefit from each other.