Final answer:
The sentence 'Yo _____ mañana.' (salir) is correctly completed with 'saldré', indicating a future intention to leave or go out.
Step-by-step explanation:
The correct form of the verb salir to complete the sentence 'Yo _____ mañana.' is saldré. 'Salir' is an irregular verb, often referred to as a GO verb, because in the first person singular present tense (yo form), it ends in -go. Therefore, when conjugated in the future tense for yo, it becomes 'saldré', which means 'I will leave' or 'I will go out' tomorrow.