**Core Concept**
The plasma membrane, also known as the cell membrane, is a thin, semi-permeable lipid bilayer that surrounds the cytoplasm of a cell, separating the internal cellular environment from the external extracellular space. This membrane is composed of various molecules, including lipids, proteins, and carbohydrates, which work together to maintain cellular homeostasis.

**Why the Correct Answer is Right**
The main constituent of the plasma membrane is phospholipids, specifically phospholipid bilayer. Phospholipid molecules have a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail, allowing them to form a bilayer that is impermeable to water and ions. This unique arrangement enables the plasma membrane to maintain its structural integrity while allowing for the selective transport of molecules in and out of the cell. The phospholipid bilayer is also dynamic, with phospholipid molecules constantly moving and interacting with each other and other membrane components.

**Why Each Wrong Option is Incorrect**
**Option A:** Glycocalyx - While the glycocalyx, a layer of glycoproteins and glycolipids, is present on the surface of the plasma membrane, it is not the main constituent of the membrane itself.
**Option B:** Proteins - Although proteins are an essential component of the plasma membrane, they are not the primary constituent. Proteins are embedded within the phospholipid bilayer and perform various functions, including transport, signaling, and cell-cell adhesion.
**Option C:** Carbohydrates - Carbohydrates, such as glycoproteins and glycolipids, are also present on the surface of the plasma membrane but are not the main constituent of the membrane.

**Clinical Pearl / High-Yield Fact**
The fluid mosaic model, proposed by S.J. Singer and Garth Nicolson in 1972, describes the structure of the plasma membrane as a dynamic, fluid bilayer composed of phospholipids and embedded proteins. This model explains the selective permeability and fluidity of the plasma membrane, which is essential for various cellular processes, including cell signaling and transport.

**Correct Answer:** C. Phospholipid bilayer.