## Core Concept
The question revolves around acid-base balance in blood, specifically focusing on a rapid method to assess acid-base composition using pCO2 levels. Acid-base balance is crucial for physiological functions, and its imbalance can lead to various clinical conditions. The method suggested likely involves interpreting blood gas values.

## Why the Correct Answer is Right
The correct answer, **D.**, corresponds to the method known as the "Winter's formula" or more accurately in this context, likely referring to an approach using the Henderson-Hasselbalch equation or a simplified version thereof, but most directly related to **D.**: The method that fits well with a quick assessment using pCO2 is the one that allows for the estimation of expected pH or bicarbonate levels based on pCO2. However, without specific formulae provided for A, B, and C, we infer based on common practices. Typically, for a quick estimation, one might use the fact that for every 10 mmHg increase in pCO2, bicarbonate (HCO3-) decreases by 2 mEq/L in a respiratory acidosis, or apply Winter's formula which is HCO3- = 0.3 * pCO2 + 10 (or similar simplified relationships).

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because, without a specific formula, it's hard to directly refute, but commonly used methods like the ones involving direct calculation of base excess or using specific acid-base nomograms are more complex than a quick "rule of thumb" involving pCO2.
- **Option B:** Similarly, this option lacks specificity but generally, methods not directly utilizing pCO2 or providing a straightforward calculation for acid-base status based on pCO2 would be less likely.
- **Option C:** This option would be incorrect if it represents a method not applicable for rapid bedside estimation of acid-base status based primarily on pCO2 levels.

## Clinical Pearl / High-Yield Fact
A useful clinical pearl is that in **respiratory acidosis**, for every 10 mmHg increase in pCO2, the bicarbonate level increases by 1 mEq/L. Conversely, in **respiratory alkalosis**, for every 10 mmHg decrease in pCO2, bicarbonate decreases by 2 mEq/L. This quick estimation helps in assessing the acid-base status and can guide the management.

## Correct Answer Line
**Correct Answer: D.**