Which of the following is characterized by its rapid spread and ability to form dense mats?

Delairea odorata, commonly known as Cape Ivy, is recognized for its rapid growth and ability to create dense mats, which can significantly impact ecosystems. This vine is known for its aggressive growth patterns, allowing it to quickly cover large areas. Its dense foliage can outcompete native flora for resources such as sunlight, water, and nutrients, leading to reduced biodiversity and altering habitats.

The other options, while they may also have invasive characteristics, do not exhibit the same rapid mat-forming behavior that Cape Ivy does. Yellow star thistle is known for its thorny nature and ability to dominate areas, but it does not typically form the dense mats characteristic of Cape Ivy. Pennisetum setaceum, or Crimson Fountain Grass, can spread rapidly but is more known for its clumping growth form rather than forming a thick, mat-like cover. Centaurea stoebe, or Spotted Knapweed, also displays invasive tendencies, but its growth does not lead to the same dense mat formation observed with Delairea odorata. Thus, Cape Ivy stands out as the species that most fits the description of rapid spread and mat formation.