Answer:
Option (a).
Step-by-step explanation:
Phylogentic tree may be defined as the the diagrammatic representation of the evolutionary relationship of the organism. The phylogenetic trees are made on the hypothesis and do not explain any fact.
The branching in the phylogenetic tree determines the evolution of the species from the common ancestor. The common ancestor is located at the base of the phylogenetic tress. Since, Ardipithecus ramidus is located at the base it shows that Ardipithecus ramidus is a common ancestor.
Thus, the correct answer is option (a).