What design technique involves the tight positioning of flower clusters at the base of an arrangement, creating rounded hills?

The design technique that involves the tight positioning of flower clusters at the base of an arrangement, creating rounded hills, is known as pillowing. This technique is characterized by a low and rounded form, where flowers are placed closely together to form a compact and full appearance. This creates a sense of depth and dimension at the base of the arrangement, giving it a visually appealing and lush look. Pillowing is often used to enhance the overall structure of a floral design while providing a soft and harmonious gradient, allowing the eye to flow naturally across the piece.

The other options pertain to different arrangement styles but don't capture the essence of the pillowing technique. Stacking typically refers to layering elements in a more structured manner, tiered arrangements involve distinct levels or layers, and grouping focuses on the clustering of like flowers without the rounded shape that pillowing achieves. Each of these different techniques has its own unique visual impact and purpose in floral design.