The appropriate reflexive verb in the conditional tense to complete the sentence "Ellas _____ todas las noches" is Se bañarían. The Option B.
In this sentence, the correct reflexive verb is "bañarían". The word in conjuction means "they would bathe" in the conditional tense.
The reflexive pronoun "se" is added to the verb to indicate that the action is being done by the subject (ellas) to themselves. So, "Ellas se bañarían todas las noches" translates to "They would bathe themselves every night."
In Spanish, the conditional tense is used to express actions that would happen under certain conditions or in the future. In this case, it implies that "they" would bathe themselves every night if certain conditions were met.