The correct answer would be the 1960s (1969 to be precise).
Explanation:
Two kingdom system was presented by the Carolus Linnaeus in his book Systema Naturae in 1735. He divided organisms into two groups Animalia and Plantae. Many biologists have found this system inadequate as it comprises diverse organisms only in two groups.
The five-kingdom system was presented by Whittaker in 1969 to replace the two kingdoms system of classification. It was based on the mode of nutrition, the complexity of cell structure and the complexity of body structure.
