Final answer:
Preload in cardiac physiology is best described by end-diastolic volume (EDV), which is the volume of blood in the ventricles right before contraction.
Step-by-step explanation:
The value that best describes preload in cardiac physiology is B. End-diastolic volume (EDV). Preload refers to the amount of blood in the ventricles at the end of atrial systole, just before ventricular contraction. A higher EDV indicates a greater preload, and it's influenced by filling time, which is the duration of ventricular diastole when filling occurs. Changes in heart rate and contractility can affect both EDV and preload. Preload is a critical factor for the heart's pumping capacity and is distinct from other measurements such as stroke volume, cardiac output, and mean arterial pressure.