**Core Concept**
The control of automatic contractility of the heart is primarily governed by the autonomic nervous system, with the sympathetic and parasympathetic divisions playing crucial roles. The **sympathetic nervous system** increases heart rate and contractility, while the **parasympathetic nervous system** has the opposite effect.

**Why the Correct Answer is Right**
The single most important factor in controlling automatic contractility of the heart is the sympathetic nervous system, which acts through **beta-1 adrenergic receptors** to increase heart rate and contractility. This is mediated by the release of **norepinephrine**, which binds to these receptors, activating a signaling cascade that ultimately leads to increased **calcium influx** into cardiac myocytes, thereby enhancing contractility.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, although important, it is not specified and thus cannot be evaluated as the most important factor.
**Option B:** Incorrect as it is not provided, and thus its relevance to contractility control cannot be assessed.
**Option C:** Incorrect because, similar to options A and B, it lacks specification, making it impossible to determine its importance in contractility control.
**Option D:** Incorrect for the same reason as options A, B, and C; it is unspecified and thus cannot be considered the most important factor.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the **sympathetic nervous system** has a predominant role in regulating heart contractility, especially under conditions of stress or physical activity, highlighting the importance of this system in clinical scenarios such as heart failure or cardiac arrhythmias.

**Correct Answer:** D. Sympathetic nervous system