Which of the following is a hybrid of Cistus?

The correct choice is a specific designation of a hybrid plant. Cistus x hybridus is classified as a hybrid due to the "x" in its nomenclature, which indicates that it is a cross between different species within the Cistus genus or between Cistus and closely related genera. This hybrid classification reflects the combination of traits from its parent species, often leading to enhanced characteristics such as flower color, growth habit, or disease resistance compared to its progenitors.

The other options represent distinct species within the Cistus genus. Cistus incanus, Cistus ladanifer, and Cistus salviifolius are all recognized species and do not possess the hybrid designation. Understanding the significance of the nomenclature in plant taxonomy is vital for identifying and classifying plants correctly. Each chosen name reveals the plant's lineage and inherent characteristics that may not be as pronounced in hybrids.