What is the correct spelling of the missing word? "No matter how hard he tried, he could never master the ______ of the difficult piano composition."

The correct spelling of the missing word is "rhythm." This word refers to a patterned recurrence of elements in music, which is essential for a piano composition. When discussing mastering a piano piece, understanding its rhythm is crucial, as it dictates the timing and flow of the music.

The other spellings presented contain errors that make them incorrect. The choice with "rithm" omits the vital initial letter that clearly indicates the term related to musical timing. "Rhthm" contains a double 'h', which does not conform to the correct structure of the word, while "rythm" incorrectly substitutes 'y' for 'h,' leading to confusion with another term. Mastering the correct spelling and understanding its implications in the context of music is key to comprehending the overall meaning of the sentence.