Final answer:
The best replacement for the underlined words is 'thrilled'.
Step-by-step explanation:
The best replacement for the underlined words in the given sentence is thrilled. It is the past tense of the verb 'thrill', which means to feel excited or happy about something. In the context of the sentence, Mariano felt thrilled to be going back to Washington, DC, indicating his excitement and happiness about the trip.
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