What concept describes the shift of metrical groupings through changes in time signature?

The correct answer, which describes the concept of a shift in metrical groupings through changes in time signature, is changing meter. This term refers to the practice where a piece of music fluctuates between different time signatures, creating a dynamic and varied rhythmic structure. As the music progresses, the underlying beats may reorganize, allowing composers to introduce complexity and interest into the piece.

In contrast, polyrhythm refers to the simultaneous use of two or more conflicting rhythms, metric modulation involves changing the tempo or pulse while maintaining the original meter, and syncopation is a rhythmic technique that emphasizes off-beats or unexpected beats within a consistent metric framework. Each of these concepts deals with rhythm and meter but does not specifically address the transition between different time signatures as changing meter does. This makes changing meter the most fitting concept for describing the shift of metrical groupings.