Carbohydrate-containing covering associated with the outer leaflet of the plasma membrane

The correct answer is option B, glycocalyx.

Explanation:
The plasma membrane is a selectively permeable barrier that surrounds the cell, separating the intracellular environment from the extracellular environment. It consists of a phospholipid bilayer with embedded proteins and other molecules. One important component of the plasma membrane is the glycocalyx.

Glycocalyx:
The glycocalyx is a carbohydrate-containing covering that is associated with the outer leaflet of the plasma membrane. It is made up of glycoproteins, glycolipids, and proteoglycans. These molecules have carbohydrate chains attached to them, forming a sugar coating on the outer surface of the cell.

Functions of the Glycocalyx:
1. Cell recognition: The glycocalyx plays a crucial role in cell-cell recognition and communication. The carbohydrate chains on the glycocalyx act as cell surface markers, allowing cells to recognize and interact with each other.
2. Protection: The glycocalyx acts as a protective barrier for the cell. It helps to prevent the entry of pathogens and toxins into the cell by acting as a physical barrier.
3. Cell adhesion: The glycocalyx is involved in cell adhesion, allowing cells to stick together and form tissues and organs.
4. Lubrication: The glycocalyx helps to reduce friction between cells and their environment. It provides a lubricating layer on the cell surface, allowing cells to move smoothly.

Other options:
a) Phospholipid: Phospholipids are the main components of the plasma membrane, but they do not contain carbohydrates. They form the lipid bilayer of the membrane.
c) Carrier protein: Carrier proteins are embedded in the plasma membrane and are involved in the transport of molecules across the membrane. They do not contain carbohydrates.
d) Band 3 protein: Band 3 protein is a specific type of protein found in red blood cells. It is involved in the transport of carbon dioxide and chloride ions. It does not contain carbohydrates and is not associated with the outer leaflet of the plasma membrane in general cell types.

In summary, the correct answer is option B, glycocalyx, which is a carbohydrate-containing covering associated with the outer leaflet of the plasma membrane. It plays important roles in cell recognition, protection, adhesion, and lubrication.