Step 1: Linnaeus’s classification system.
Carlous Linnaeus, a Swedish botanist, is credited with being the first to develop a modern classification system for plants and animals. His system of binomial nomenclature and hierarchical classification laid the foundation for how organisms are grouped.
Step 2: Other contributions.
Later, Bentham & Hooker proposed a broader classification scheme, and R.H. Whittaker proposed a 5-kingdom classification system that included Monera, Protista, Fungi, Plantae, and Animalia. However, Linnaeus was the first to develop a classification system for plants.
Step 3: Conclusion.
Thus, Carlous Linnaeus was the first to classify the plant kingdom, corresponding to option (a).