## Core Concept
The primary form of cortisol in plasma is bound to proteins, mainly **corticosteroid-binding globulin (CBG)**, also known as transcortin, and to a lesser extent to **albumin**. This binding affects the biological activity and transport of cortisol in the bloodstream.

## Why the Correct Answer is Right
Cortisol, a steroid hormone produced by the adrenal cortex, circulates in the plasma primarily bound to **corticosteroid-binding globulin (CBG)**. This binding is specific and high-affinity, with about 90-95% of cortisol bound to CBG, and the rest bound to **albumin**. Only the **unbound fraction** of cortisol (about 5-10%) is biologically active. The binding to CBG helps in the transport of cortisol to its target tissues and also plays a role in regulating its plasma levels.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because, although albumin does bind cortisol, it is not the primary binding protein for cortisol in plasma.
- **Option B:** This option is incorrect because it does not accurately represent the primary form of cortisol in plasma.
- **Option D:** This option is incorrect because free cortisol, although biologically active, represents only a small fraction of total cortisol in plasma.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **only the unbound fraction of cortisol is biologically active**. Conditions that alter the levels of CBG or albumin can affect the total cortisol levels measured in the blood but not necessarily the level of biologically active cortisol.

## Correct Answer: C. Bound to Corticosteroid-Binding Globulin (CBG).