Final answer:
A polyphyletic group is formed when two or more clades are combined based on a shared feature, but this grouping excludes the common ancestor and other members that would unite them into a single clade.
Step-by-step explanation:
A clade, also known as a monophyletic group, is a group of organisms that share a common ancestor and all of its descendants. When two or more clades are combined into one group based on a shared feature, but this grouping excludes the common ancestor and other members that would unite them into a single clade, it is termed a polyphyletic group (option b).
For example, let's consider the grouping of birds, bats, and insects into a single group based on having wings. While these organisms all have wings, they do not share a common ancestor with wings, as the wings in birds are a derived trait from their common ancestor with dinosaurs, while the wings in bats and insects evolved independently. Therefore, this grouping would be considered a polyphyletic group.