**Core Concept**
The question is testing the understanding of the slow excitatory postsynaptic potential (EPSP) in the sympathetic ganglion, specifically the receptor involved in the postganglionic neuron. This involves the neurotransmitter acetylcholine and its interaction with muscarinic receptors.

**Why the Correct Answer is Right**
The slow EPSP in the sympathetic ganglion is mediated by the activation of muscarinic receptors on the postganglionic neuron. This is due to the release of acetylcholine from the preganglionic neuron, which then binds to M2 muscarinic receptors on the postganglionic neuron. The activation of these receptors leads to a decrease in the excitability of the postganglionic neuron, resulting in a slow EPSP. This is in contrast to the nicotinic receptors, which are involved in the fast EPSP.

**Why Each Wrong Option is Incorrect**
**Option A:** Nicotinic receptors are involved in the fast EPSP in the sympathetic ganglion, not the slow EPSP.
**Option B:** There is no specific receptor subtype that is directly associated with the slow EPSP in the sympathetic ganglion.
**Option C:** This option is not relevant to the question and is a distractor.

**Clinical Pearl / High-Yield Fact**
The muscarinic receptors play a crucial role in the regulation of the sympathetic ganglion, and their activation can lead to a decrease in the excitability of the postganglionic neuron.

**Correct Answer:** M2. Muscarinic receptors (M2 subtype) are involved in the slow EPSP in the sympathetic ganglion.