Which of the following best defines 'mass flowers' in floral design?

Mass flowers are characterized by their ability to create fullness and volume within a floral arrangement. These types of flowers typically have a large bloom size and a dense growth habit, which enables them to occupy significant space visually. Their rounded shapes and mass presence allow a designer to build depth and add a focal point to the overall design.

In contrast, flowers that have long, slender stems are often used for height and can lend an airy feel rather than fullness. Flowers used solely as accents serve a different purpose, typically intended to complement the main elements without dominating the design. Lastly, flowers that outline the arrangement are generally used to create structure and define the shape rather than to create the bulk that mass flowers provide. Therefore, the defining characteristic of mass flowers as those that create a full, rounded effect makes this choice the most appropriate in the context of floral design.