Which of these is known as Rockspray Cotoneaster?

Rockspray Cotoneaster is indeed known scientifically as Cotoneaster horizontalis. This plant is a deciduous shrub characterized by its low, spreading growth habit, which makes it an excellent ground cover. It features small, glossy leaves and produces clusters of pink or white flowers that can later give way to bright red berries, attracting birds and providing ornamental interest in the landscape.

The horizontal growth form of Cotoneaster horizontalis is particularly notable; it often trails over rocks and walls, hence the common name "Rockspray." This adaptability to various soils and its ability to thrive in less-than-ideal conditions further contribute to its popularity in landscaping.

Other plant options listed in the question belong to different genera and families and do not share the same characteristics as Cotoneaster horizontalis. Thus, among the choices presented, B is the only one that correctly corresponds to the common name Rockspray Cotoneaster.