To conduct a phylogenetic analysis, we need an outgroup to:

a. determine which character states are ancestral and which are derived.
b. determine which characteristics to include in our analysis.
c. decide which characters are analogous and which are homologous.
d. decide which molecular data to use.

a. determine which character states are ancestral and which are derived.

Explanation:

Phylogenetic analysis is a scientific classification of species based solely on the relationships of evolutionary proximity between different species, reconstructing the history of their diversification (phylogenesis) from the origin of life on earth to the present.

In all cases it is advisable to use different methods of phylogenetic reconstruction, as well as the comparative analysis of the topology of the different phylogenetic trees and the calculation of the robustness of each of them.

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