Which of the following best defines a clade?

A clade is defined as a group that includes a common ancestor and all of its descendants. This definition is essential in understanding evolutionary relationships among organisms. Clades represent distinct branches on the tree of life, illustrating how species evolve and diverge from common ancestors over time.

In cladistics, which is a method of biological classification, organisms are grouped based on shared derived characteristics that come from their most recent common ancestor. This means that for any given clade, you not only identify the ancestor but also ensure that all lineage members that descended from that ancestor are included in the clade. This characteristic makes clades a crucial concept in evolutionary biology and helps biologists to construct phylogenetic trees to visualize evolutionary relationships.

The other options do not accurately reflect the definition of a clade. They either suggest exclusivity to descendants without referencing a common ancestor or imply randomness, which undermines the systematic nature of clade classification.