The answer is commensalism.
Commensalism is a relationship between two
species in which only one of them has benefit, and the other one is not
affected.
In this relationship, frogs benefit from this relationship. They get water and shelter from the Bromeliad. The Bromeliad does not gain or lose anything in the relationship, so it is not affected anyway. Thus, the relationship between frogs and the Bromeliad is an example of commensalism.
