Which of the following plans could states adopt individually as allowed by the Constitution?

The correct answer, the proportional plan, reflects a method by which states can allocate their electoral votes in a way that aligns with the popular vote within that state for presidential elections. The U.S. Constitution grants states the authority to determine how they will appoint their electors, allowing them the flexibility to adopt various systems.

The proportional plan, in particular, is one that allows for a more representative distribution of electoral votes based on the share of votes each candidate receives. This contrasts with other systems like winner-takes-all or direct election that may not offer the same level of representation to minority choices in a presidential election.

While states have significant latitude in how they approach the electoral vote process, the winner-takes-all system is employed by most states but is not the only permissible one, making the proportional option valid. States can also implement rules surrounding majority voting but that doesn't pertain to how electoral votes are allocated directly.

In summary, the ability of states to adopt a proportional plan acknowledges the Constitution's provision for tailored electoral vote processes, which can enhance voter representation.