**Core Concept:**
The question is asking about a major mineral that plays a crucial role in forming and maintaining the structure of biological membranes. Biological membranes are essential components of cells, enclosing the cell and regulating the passage of substances in and out. These membranes are made up of lipids, proteins, and carbohydrates, which provide a selective barrier for the cell. The mineral in question contributes to the structural integrity of these membranes.

**Why the Correct Answer is Right:**
The correct answer is **D**: Cholesterol. Cholesterol is a steroid molecule that serves several crucial functions in biological membranes:

1. Cholesterol forms lipid rafts, which are micro domains within the membrane that play a role in signal transduction, vesicle trafficking, and immune response modulation.
2. Cholesterol helps maintain membrane fluidity, ensuring the membrane's ability to adapt to varying environmental conditions, such as temperature changes.
3. Cholesterol contributes to the structural stability of the lipid bilayer, preventing it from collapsing or losing its integrity.

**Why Each Wrong Option is Incorrect:**
1. Option A (Sodium): Sodium is a cation (positively charged ion) that plays a role in maintaining fluid balance, nerve impulse transmission, and muscle contraction. It does not directly contribute to the structural integrity of biological membranes.
2. Option B (Magnesium): Magnesium is an essential mineral for various enzymatic reactions, neuromuscular function, and bone mineralization. It is not a component of biological membranes.
3. Option C (Calcium): Calcium is essential for various physiological processes, including muscle contraction, nerve transmission, and blood clotting. Like magnesium, calcium is not a component of biological membranes.

**Clinical Pearl:**
Understanding the role of cholesterol in biological membranes is crucial for understanding cellular functions and diseases. For example, high levels of low-density lipoprotein (LDL) cholesterol can lead to the formation of atherosclerotic plaques in the arteries, contributing to cardiovascular diseases. On the other hand, high levels of high-density lipoprotein (HDL) cholesterol help remove excess cholesterol from the bloodstream, promoting cardiovascular health.

**Correct Answer: D (Cholesterol)**

Cholesterol is a sterol, similar to steroids, that is an integral part of biological membranes. It helps maintain membrane fluidity, stability, and the formation of lipid rafts, which are micro-domains within the membrane that influence signal transduction and protein function. Additionally, cholesterol is involved in the synthesis of steroid hormones and bile acids.