**Core Concept:** The freezing point of normal human plasma is influenced by the presence of proteins, particularly albumin and globulins. These proteins help in lowering the freezing point of plasma by reducing the ice nucleation process.

**Why the Correct Answer is Right:** The correct answer, **D**, refers to the freezing point of normal human plasma with 7% dextran added. Dextran is a large, water-soluble carbohydrate molecule that acts as a cryoprotectant. By binding to plasma proteins, dextran reduces the concentration of proteins available to lower the freezing point. This results in a higher freezing point of plasma, indicating that dextran has been added.

**Why Each Wrong Option is Incorrect:**

A. **Freezing point of water:** The freezing point of pure water is -40°C (-40°F), which is lower than the correct answer. Plasma's freezing point is affected by its protein content.

B. **Freezing point of pure plasma:** The freezing point of pure human plasma is approximately -45°C (-49°F), which is lower than the correct answer. The presence of proteins in plasma contributes to the lowering of its freezing point.

C. **Freezing point of pure albumin:** Albumin is one of the major proteins in plasma, but it is not solely responsible for lowering the freezing point. Other plasma proteins, such as globulins, also contribute to the lowering of the freezing point.

**Clinical Pearl:** The understanding of plasma proteins' role in lowering its freezing point is important in various clinical scenarios. For example, in perfusion of organs during cold storage, the addition of cryoprotectants like dextran helps maintain the organ's physiological state and reduces the risk of cellular damage upon reperfusion.