Whittaker's system, introduced in 1969, classifies all living organisms into five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. This classification is primarily based on:

1. Presence or Absence of a Well-defined Nucleus (Option A): This criterion distinguishes Prokaryotes (Monera, lacking a well-defined nucleus) from Eukaryotes (Protista, Fungi, Plantae, and Animalia, all having a well-defined nucleus).
2. Mode of Nutrition (Option C): Organisms are classified based on their nutritional strategies. For example, plants (Plantae) are autotrophic, fungi (Fungi) are primarily saprophytic, and animals (Animalia) are heterotrophic.
3. Complexity of Body Organization (Option D): This involves classifying organisms based on their cellular organization (unicellular vs multicellular), body structure, and differentiation.

However, the mode of reproduction (Option B) is not a primary criterion in Whittaker's system. While reproductive methods are important in biological classification, they are not the foundational basis of the five-kingdom model.