Final answer:
The most appropriate verb to fill in the blank is 'hace', which means 'I have been waiting' in English.
Step-by-step explanation:
The most appropriate verb to fill in the blank is hace, which means 'I have been waiting' in English. The phrase 'tres horas que espero' translates to 'I have been waiting for three hours.' In this sentence, the verb 'hace' is used to express the duration of the action of waiting.
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