A monophyletic group would be best described as:
b) a grouping of all species descended from a common ancestor including that ancestor.
This definition is important in the study of phylogenetics, as a monophyletic group, also known as a clade, includes an ancestor and all its descendants. This is fundamental to understanding evolutionary relationships among species. In contrast, options such as ‘most but not all species’ or simply ‘species descended from’ do not accurately capture the essence of monophyly, as they might leave out crucial parts of the lineage or fail to include the common ancestor itself. To illustrate, think of a family tree where a parent and all their children form a complete branch; that entire branch represents a monophyletic group.