The only verb that fits the impersonal necessity haber construction of this sentence is the infinitve bañar. Bañaba and baña are in the wrong tenses, and voltar, while in the right tense, makes no sense in context.

Ser is the only correct answer as it is both the only verb that agrees grammatically with the adjective libre. Further, the infintive structure is neccesary as the verb's indicative form, soy, would be incorrect in an impersonal statement.

The correct answer is salir. Because of the "de" the following word must be in the infinitive form.