**Core Concept**
Preload refers to the initial stretching of the cardiac myocytes and the subsequent increase in ventricular volume prior to contraction. It is a critical determinant of the end-diastolic volume and the stroke volume of the heart.

**Why the Correct Answer is Right**
During diastole, the ventricle relaxes and fills with blood from the atrium. The increased volume of blood in the ventricle stretches the cardiac myocytes, which activates the stretch-activated channels and increases the intracellular calcium concentration. This increased calcium concentration triggers the contraction of the cardiac myocytes and the ejection of blood into the aorta. Therefore, the correct answer is related to the increased volume of blood in the ventricle.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because preload is not directly related to the afterload, which is the resistance to blood flow that the ventricle must overcome to eject blood into the aorta.
**Option B:** This option is incorrect because the Frank-Starling mechanism is related to the increase in stroke volume in response to an increase in preload, but it is not the correct answer.
**Option C:** This option is incorrect because the compliance of the ventricle refers to its ability to stretch and fill with blood, but it is not the correct answer.

**Clinical Pearl / High-Yield Fact**
Remember that the Frank-Starling mechanism is a critical determinant of the stroke volume and the cardiac output. It is a positive feedback loop that increases the stroke volume in response to an increase in preload.

**Correct Answer: B. The Frank-Starling mechanism.**