Question

the main trunk at the bottom of a phylogenetic tree represents the most successful members of the lineage. the extinct members of the lineage. the earliest common ancestors of all the organisms in the tree. the organisms for which most fossils have been discovered.

Explanation:

A phylogenetic tree shows evolutionary relationships. The main trunk at the bottom is the root, which represents the earliest common ancestor. It's not about success (success is relative and not shown by the trunk), not just extinct members (could have living descendants), and not about fossil discovery quantity (fossil record is incomplete and not the basis of the trunk's meaning).

Answer:

the earliest common ancestors of all the organisms in the tree.