Final answer:
The canon that suggests an association between disease frequency and causative factor potency is C. Concomitant variation.
Step-by-step explanation:
The canon that suggests there is an association between frequency of disease and the potency (virulence) of a causative factor or exposure is C. Concomitant variation. This canon indicates that as the intensity of the causative factor increases, so does the incidence of the disease.
In terms of Mars' four canons, neither A. Difference, B. Agreement, nor D. Residues directly address the association between frequency of disease and the potency of a causative factor as clearly as Concomitant variation does.