## Core Concept
The primary buffer system in interstitial fluid, which is the fluid surrounding cells in the body, is crucial for maintaining acid-base balance. Buffer systems help neutralize excess hydrogen or hydroxide ions, thereby maintaining the body's pH within a narrow range. The main buffer systems in the body include the bicarbonate buffering system, phosphate buffer system, and protein buffer system.

## Why the Correct Answer is Right
The bicarbonate buffer system, which includes **bicarbonate ions (HCO3-)** and **carbonic acid (H2CO3)**, is the most important buffer in interstitial fluid. This system works as follows:
- Carbonic acid (H2CO3) can donate a proton (H+), acting as an acid: H2CO3 → H+ + HCO3-
- Bicarbonate (HCO3-) can accept a proton, acting as a base: H+ + HCO3- → H2CO3
This buffer system is particularly effective in interstitial fluid because it can easily be regulated through respiratory (by changing CO2 levels, which affects H2CO3 formation) and renal mechanisms (by adjusting HCO3- reabsorption or excretion).

## Why Each Wrong Option is Incorrect
- **Option A:** While phosphate buffers are important, especially in intracellular fluid and renal regulation of acid-base balance, they are not the primary buffer system in interstitial fluid.
- **Option B:** This option seems to refer to a buffer system but is not clearly defined. However, given the context, it's likely less relevant than the bicarbonate buffer system in interstitial fluid.
- **Option D:** Proteins can act as buffers, especially in blood (e.g., hemoglobin and plasma proteins) and intracellularly, but they are not the most important buffer system in interstitial fluid.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the **bicarbonate buffering system** is crucial for maintaining acid-base balance in the body and is effectively managed through respiratory and renal compensatory mechanisms. This system is particularly vital in clinical settings where acid-base disturbances are common, such as in diabetic ketoacidosis or respiratory failure.

## Correct Answer Line
**Correct Answer: C. HCO3-/H2CO3.**