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A phylogenetic tree is a graphical or visual depiction of one or more species' evolutionary history.

Option (a) is represented by a single ancestor. A phylogenetic tree's trunk is used to symbolize a single common ancestor that split into two species at the node. As a result, option is the proper response (a).

The pool of ancestral creatures is specified as option (b). The pool of ancestral creatures is represented by the complete tree preceding the species under investigation, not just the stem. Option (b) is thus erroneous.

Option (c) has been designated as a new species. After the trunk has split into two or more subgroups to end up at a given point, new species are identified. As a result, choice (c) is erroneous.

Option (d) is referred to as an older species. The trunk is used to refer to the common ancestor of the species that have diverged from it, not merely the ancient species. Option (d) is thus erroneous.

A phylogenetic tree's stem represents the common ancestor, which split into two or more species at a later point in time.