Significant literature has developed as followers of Kant have constructed devices to make the formula work as a/an ______.

A) justification
B) rule
C) action guide
D) anterior motive

The categorical imperative, which is the formula to determine if an action is morally right or wrong, can be seen as an action guide that provides guidance for individuals in making ethical decisions. This is supported by the fact that Kant himself described it as a "practical rule" for moral decision-making. Therefore, the best choice is C, as it accurately represents the purpose and function of the categorical imperative. A and B are not as comprehensive in describing the formula's role and D is not a relevant option.