**Core Concept**
Ligand-gated ionic channels are a type of ion channel that opens or closes in response to the binding of a specific ligand, such as a neurotransmitter. This allows for the rapid transmission of signals across synapses in the nervous system. Ligand-gated channels are a crucial component of synaptic transmission and are involved in various physiological processes, including muscle contraction, neuronal excitability, and learning and memory.

**Why the Correct Answer is Right**
The mechanism of ligand-gated channels involves the binding of a ligand to a specific receptor site, which triggers a conformational change in the channel protein. This change opens the channel, allowing ions to flow through and generating an electrical signal. For example, the nicotinic acetylcholine receptor is a ligand-gated channel that responds to the binding of acetylcholine, a neurotransmitter released by motor neurons. When acetylcholine binds to the receptor, it triggers the opening of the channel, allowing sodium and potassium ions to flow through and generating an excitatory postsynaptic potential (EPSP).

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a type of ion channel that responds to a ligand. While some ion channels are voltage-gated or mechanically gated, ligand-gated channels are a distinct class of channels that respond to the binding of a specific ligand.
**Option B:** This option is incorrect because it describes a type of ion channel that is not specifically ligand-gated. While some ion channels are voltage-gated, they do not respond to the binding of a ligand.
**Option C:** This option is incorrect because it describes a type of receptor that is not an ion channel. G protein-coupled receptors (GPCRs) are a type of receptor that responds to the binding of a ligand, but they do not form a channel in the plasma membrane.

**Clinical Pearl / High-Yield Fact**
Ligand-gated channels are the target of many drugs, including anesthetics, muscle relaxants, and anticonvulsants. Understanding the mechanism of ligand-gated channels is crucial for the development of effective treatments for various neurological and psychiatric disorders.

**Correct Answer:** A. Ligand-gated channels.