What does "key modulation" refer to in music?

Key modulation refers specifically to the process of changing from one key to another within a piece of music. This technique is commonly used to create variety and to develop musical ideas further, adding emotional depth and dynamics to a composition. By modulating, a composer can shift the tonal center of the music, which can evoke different feelings and highlight particular sections of the piece.

In contrast, the other options pertain to different aspects of musical performance and composition, such as tempo changes, instrument variations, or stylistic shifts, but do not encompass the concept of modulation between keys. Therefore, the focus on key modulation as a key component in music theory is what makes this answer the most accurate.