**Core Concept**
Parasympathetic stimulation of the cardiovascular system involves the release of acetylcholine (ACh) at muscarinic receptors, leading to a decrease in heart rate, cardiac contractility, and vascular tone. This results in a decrease in cardiac output and peripheral resistance.

**Why the Correct Answer is Right**
The parasympathetic nervous system exerts its effect on the heart through the vagus nerve, which releases ACh at muscarinic receptors, specifically M2 receptors in the sinoatrial (SA) node. This leads to a decrease in the firing rate of the SA node, resulting in a decrease in heart rate. Additionally, parasympathetic stimulation causes the heart to contract less forcefully, reducing cardiac output. Vascular tone is also decreased due to the release of ACh, leading to vasodilation and a decrease in peripheral resistance.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because parasympathetic stimulation does not increase heart rate, it decreases it.
**Option B:** This option is incorrect because parasympathetic stimulation does not increase cardiac contractility, it decreases it.
**Option C:** This option is incorrect because parasympathetic stimulation does not increase vascular tone, it decreases it.

**Clinical Pearl / High-Yield Fact**
The parasympathetic nervous system has a significant impact on the cardiovascular system, and its effects can be seen in conditions such as bradycardia and vasovagal syncope. Understanding the effects of parasympathetic stimulation is crucial in managing patients with cardiovascular disease.

**Correct Answer: D.