Which cultivar is known as Thundercloud Plum?

The cultivar known as Thundercloud Plum is indeed Prunus cerasifera 'Thundercloud'. This specific cultivar is recognized for its beautiful ornamental features, notably its striking purple foliage and small pink flowers that bloom in spring. It is a popular choice for landscape use due to its aesthetic appeal and adaptability.

Unlike the other options, which refer to different species or cultivars not associated with the name "Thundercloud," Prunus serrulata 'Kwanzan' is commonly known as the Kwanzan Cherry and is recognized for its double-pink blossoms, while Prunus subhirtella 'Pendula', known as the Weeping Higan Cherry, features a cascading form and delicate flowers. Oxydendrum arboreum, also known as sourwood, is quite different botanically, as it is a tree valued for its fall color and floral display rather than resembling a plum tree in foliage or fruit characteristics.

Therefore, Prunus cerasifera 'Thundercloud' is accurately named and represents the correct answer due to its specific identification and ornamental attributes.