To solve this problem, we need to understand the basis of the five kingdom system of classification suggested by R.H. Whittaker.$\newline$Whittaker's classification is based on the following criteria:$\newline$• Mode of nutrition: Organisms are classified based on how they obtain their food, such as autotrophic or heterotrophic.$\newline$• Complexity of body organization: This refers to the structural complexity, ranging from unicellular to multicellular organisms.$\newline$• Mode of reproduction: This includes the method of reproduction, whether sexual or asexual.$\newline$• Phylogenetic relationships: This involves the evolutionary history and relationships among organisms.$\newline$The presence or absence of a well-defined nucleus is not a criterion used by Whittaker in his five kingdom classification.$\newline$Instead, the presence or absence of a nucleus is a criterion used in the two-empire system, which distinguishes between prokaryotes and eukaryotes.$\newline$Therefore, the correct option is Option 1: Presence or absence of a well-defined nucleus.