**Core Concept**
The question is testing the distribution of blood volume in the human body. Blood volume is the amount of blood present in the circulatory system, and it is an essential parameter in understanding various physiological and pathological conditions.

**Why the Correct Answer is Right**
The correct answer is **D. Plasma** because a significant portion of blood volume is occupied by plasma, which is the liquid component of blood. Plasma makes up approximately 55% of the total blood volume and contains water, proteins, nutrients, waste products, and various ions and hormones. The remaining 45% of blood volume is composed of formed elements, including red blood cells, white blood cells, and platelets.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because blood cells, including red blood cells, white blood cells, and platelets, make up only 45% of the total blood volume, not the majority.

**Option B:** This option is incorrect because blood cells are a component of blood, but they do not represent the majority of blood volume.

**Option C:** This option is incorrect because blood vessels are part of the circulatory system, but they do not contain blood; they are the channels through which blood flows.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that blood volume is a critical parameter in various clinical conditions, such as shock, trauma, and hemorrhage. Understanding the distribution of blood volume can help clinicians diagnose and manage these conditions effectively.

**Correct Answer: D. Plasma**