**Core Concept**
Preload refers to the initial stretching of the cardiac myocytes prior to contraction, which is directly related to the volume of blood in the ventricles at the end of diastole. Increasing preload allows the heart to pump more effectively by increasing the stroke volume.

**Why the Correct Answer is Right**
The correct answer is related to the Frank-Starling mechanism, which describes the relationship between the end-diastolic volume (preload) and the stroke volume. According to this mechanism, an increase in preload leads to an increase in stroke volume, as the cardiac myocytes are stretched, allowing them to contract more forcefully. This is mediated by the troponin-tropomyosin complex and the sliding filament theory.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because decreasing afterload (not increasing preload) would be the desired effect in conditions such as pulmonary hypertension.

**Option B:** This option is incorrect because increasing contractility (inotropic state) would not directly increase preload, although it may increase stroke volume.

**Option C:** This option is incorrect because increasing heart rate (tachycardia) would decrease the diastolic filling time, thus decreasing preload.

**Clinical Pearl / High-Yield Fact**
A key concept to remember is that the Frank-Starling mechanism is a preload-dependent mechanism, meaning that increasing preload will increase stroke volume, but only up to a point, beyond which it may lead to decreased contractility and efficiency.

**Correct Answer: D. Increasing venous return**