What is the appropriate word to fill in the blank: It was a beautiful performance. We all watched her from a balcony in the theater as she danced gracefully across the ________ stage.

The word "massive" is the appropriate choice to fill in the blank because it conveys a sense of grandeur and spaciousness in describing the stage where the performance took place. In the context of the sentence, which mentions a "beautiful performance" and "danced gracefully," using "massive" enhances the imagery of the scene, suggesting that the dancer's graceful movements were accentuated by a large and impressive stage.

This choice sets a fitting backdrop for the performance, emphasizing the scale and visual impact of the setting. In contrast, the other options do not align well with the tone or context of the sentence. "Stunningly" does not fit grammatically as it is an adverb, not a noun. "Lonely" introduces an emotional state that contradicts the joyous atmosphere of a beautiful performance. "Misery" carries negative connotations that are inconsistent with the overall sentiment of enjoyment and admiration being expressed.