In which context does "Horizontal Hemiola" occur?

Horizontal hemiola is a rhythmic concept that occurs specifically in passages of rhythmic complexity where different rhythmic patterns are layered on top of one another. It involves the interplay between two contrasting rhythms that create a feeling of tension and release. Typically, this occurs when a piece of music in a 3/4 time signature is played or perceived in a way that emphasizes groups of two beats, creating a sense of 'feeling' in a different metric grouping than what is written.

In this context, the complexity arises as the listener begins to perceive the music in both the original rhythmic pattern and the newly imposed structure simultaneously, leading to an interesting layering effect. This technique is often used in various musical traditions, including Western classical music and some folk genres, to draw attention to specific moments in the music.

The other options, while relating to music, do not specifically capture the essence of what horizontal hemiola entails. Shifts in tempo, melodic variation, and harmonic progression do not fundamentally focus on the rhythmic layering and complexity that characterizes horizontal hemiola. Thus, the context of rhythmic complexity is unique and foundational to understanding how and where horizontal hemiola occurs.