The word that is missing in the sentence is "a" because it is a preposition that relates the action of seeing with a person "Rafael".
What is a preposition?
The preposition is a type of word that is characterized by preceding its complement. Generally, prepositions have the function of introducing mandatory complements.
According to the above, it can be inferred that the preposition "a" is the missing word in the sentence because it is said that Monica is happy because she is going to "ver a Rafael"
Learn more about prepositions in: https://brainly.com/question/4956879
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