Which of the following is a type of ornamental grass?

Mondo grass is indeed a type of ornamental grass, specifically under the genus Ophiopogon. This plant is beloved in landscaping for its low-growing, dense foliage, and it typically features dark green or variegated leaves. It thrives in shaded areas and is often used as a ground cover or border plant due to its attractive appearance throughout the seasons.

The other options represent different types of plants. Beebalm is a flowering perennial known for attracting pollinators, particularly bees, hence its name, but it does not belong to the grass family. Paeonia sp., or peonies, are flowering plants that produce large, often fragrant blooms and are not classified as grasses. Similarly, Phlox sp. includes various flowering species commonly used in gardens, but they, too, are not grasses. Thus, the unique characteristics of Mondo grass align with the criteria for ornamental grasses, differentiating it from the other plant choices.