Final answer:
An increase in contractility is demonstrated on a Frank-Starling diagram by increased cardiac output for a given end-systolic volume.
Step-by-step explanation:
In a Frank-Starling diagram, an increase in contractility is demonstrated by increased cardiac output for a given end-systolic volume. Contractility refers to the force of the contraction of the heart muscle, and increasing contractility leads to a more forceful contraction, resulting in increased stroke volume and cardiac output. Therefore, option (B) increased cardiac output for a given end-systolic volume represents an increase in contractility on a Frank-Starling diagram.