**Core Concept**
Acetylcholine (ACh) is a neurotransmitter that acts on both nicotinic and muscarinic receptors. At nicotinic receptors, ACh plays a significant role in the transmission of signals in the peripheral nervous system, including neuromuscular junctions.

**Why the Correct Answer is Right**
When acetylcholine binds to nicotinic receptors, it facilitates the opening of sodium channels, leading to an influx of sodium ions into the muscle cell. This depolarization of the muscle cell membrane triggers an action potential, resulting in muscle contraction. This process is crucial for the transmission of signals from motor neurons to skeletal muscles at the neuromuscular junction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as nicotinic receptors are not typically associated with smooth muscle contraction.
**Option B:** This option is incorrect as acetylcholine acting on muscarinic receptors leads to smooth muscle contraction, not nicotinic receptors.
**Option C:** This option is incorrect as nicotinic receptors are not involved in the regulation of glandular secretion.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that nicotinic receptors are primarily involved in the transmission of signals at the neuromuscular junction, while muscarinic receptors are involved in smooth muscle contraction and glandular secretion.

**Correct Answer: A. Muscle contraction**