Final answer:
The best word to fill in the blank is 'accomplished' as it accurately describes Julia Myra Hess as a skilled musician.
Step-by-step explanation:
The sentence describes Julia Myra Hess as a successful musician prior to her debut in the United States in 1922. To fill in the blank, we need to choose a word that means skilled or accomplished. The best choice among the given options is accomplished. This word fits the context and accurately portrays Julia Myra Hess as an established concert pianist in England.
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