**Core Concept**
The endocrine system relies on a complex interplay of hormones to regulate various physiological processes. The specificity of communication between endocrine glands and target cells is crucial for precise physiological control. This specificity is achieved through the interaction of hormones with specific receptors on target cells.

**Why the Correct Answer is Right**
The specificity of endocrine communication is primarily determined by the unique structure of hormone-receptor interactions. Hormones, such as thyroid hormone, cortisol, and insulin, bind to specific receptors on target cells, triggering a cascade of intracellular signaling events. The binding of a hormone to its receptor activates a conformational change in the receptor, which in turn activates various downstream signaling pathways. This precise interaction between the hormone and its receptor ensures that the correct physiological response is elicited in the target cell.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the primary mechanism of specificity in endocrine communication. While transport proteins may play a role in hormone binding and transport, they are not responsible for determining the specificity of hormone-receptor interactions.

**Option B:** This option is incorrect because it is too broad and does not specifically address the mechanism of specificity in endocrine communication. While it is true that hormones can have different effects on different tissues, this is a consequence of the specificity of hormone-receptor interactions rather than the primary mechanism of specificity itself.

**Option C:** This option is incorrect because it is not directly related to the specificity of endocrine communication. Hormone degradation is an important process that affects hormone half-life and activity, but it is not responsible for determining the specificity of hormone-receptor interactions.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the specificity of hormone-receptor interactions is a critical determinant of endocrine system function. This specificity is achieved through the unique structure of hormone-receptor interactions, which ensures that the correct physiological response is elicited in the target cell. Understanding this concept is crucial for appreciating the complex interplay of hormones in regulating various physiological processes.

**Correct Answer: D.**