Final answer:
The most important aspect for classifying organisms using cladistics is shared derived characters, as they indicate recent evolutionary divergences. It's also crucial to differentiate between homologous and analogous traits to accurately construct phylogenetic trees.
Step-by-step explanation:
When using a cladistic approach to systematics, the most important aspect for classification is b) shared derived characters. These are traits that are present in some members of a group but not all, indicating a divergence in their evolutionary paths. In contrast, shared ancestral characters are traits found in all members of a group and do not provide information on more recent evolutionary relationships.
Furthermore, in the context of phylogenetic trees, it's critical to distinguish between homologous and analogous characteristics. Homologous characteristics stem from a common evolutionary history, whereas analogous traits may serve similar functions but evolve independently. This differentiation is key since homologous characteristics can be used to build phylogenetic trees, which are models that depict the evolutionary relationships based on shared evolutionary history, both physical and genetic evidence.