Answer:
The answer is A, to serve as an outgroup.
Step-by-step explanation:
The study of any phylogenetic group requires that there must be an outgroup that can serve to understand which characters are plesiomorphic (or ancestral) and how they changed over the years to a new evolved character. In this case, Lily, since it is not a member of the group which is under study (orchid), serves as an outgroup.