The Three Domain System, proposed by Woese and others, is an evolutionary model of phylogeny based on differences in the sequences of nucleotides in the cell's ribosomal RNAs (rRNA), as well as the cell's membrane lipid structure and its sensitivity to antibiotics. Comparing rRNA structure is especially useful.

The mode of nutrition refers to the way organisms obtain their energy and nutrients. There are two main modes of nutrition: autotrophic and heterotrophic.

- Autotrophic organisms are capable of synthesizing their own food using inorganic substances such as carbon dioxide and sunlight. They include plants, algae, and some bacteria.
- Heterotrophic organisms, on the other hand, rely on organic compounds produced by other organisms for their nutrition. They include animals, fungi, and some bacteria.


The presence or absence of a cell wall is another characteristic that can be used to classify organisms. A cell wall is a rigid structure that surrounds the cell membrane and provides support and protection.

- Organisms with a cell wall can be further classified into two groups based on the composition of their cell wall:
- Bacteria have a cell wall made of peptidoglycan.
- Fungi have a cell wall made of chitin.

- Organisms without a cell wall include animals and some bacteria.


The 16S rRNA gene is a highly conserved gene found in all living organisms. It codes for a small subunit of the ribosome and is commonly used in molecular biology studies to determine evolutionary relationships between organisms.

- By comparing the sequence of the 16S rRNA gene, scientists can identify similarities and differences between organisms and classify them into different groups.


All organisms have 16S rRNA genes as they are essential for protein synthesis. However, the presence or absence of these genes is not used as a criterion for classification.


Based on the given options, the most appropriate criterion for clustering the six kingdoms into three domains would be the sequence of 16S rRNA genes. This is because the 16S rRNA gene is present in all organisms and its sequence can provide valuable information about evolutionary relationships. The mode of nutrition and presence or absence of a cell wall are important characteristics for classification but may not be sufficient to accurately cluster the six kingdoms into three domains.