Final answer:
The most appropriate verb to use is 'Hace'.
Step-by-step explanation:
The most appropriate verb to fill in the blank is Hace.
The sentence 'tres horas que espero' translates to 'I have been waiting for three hours'. In Spanish, the verb 'hace' is used to express the passage of time when it refers to the action of waiting, as in this case.
Example: 'Hace tres horas que espero el autobús.' (I have been waiting for the bus for three hours.)
Learn more about Verb usage in Spanish