## **Core Concept**
The problem involves calculating the variance of a set of plasma volumes given the mean and standard deviation. Variance is a measure of dispersion or variability in a set of data, and it is calculated as the square of the standard deviation.

## **Why the Correct Answer is Right**
The standard deviation (SD) of the plasma volumes is given as 0.25 litres. The variance is calculated by squaring the standard deviation. Therefore, the variance = SD^2 = 0.25^2 = 0.0625.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not match the calculation of variance as the square of the standard deviation.
- **Option B:** This option is incorrect for the same reason as Option A; it does not correctly represent the variance.
- **Option C:** This is actually the correct calculation for variance (0.25^2), but let's evaluate all options.
- **Option D:** This option is incorrect as it does not align with the correct calculation of variance.

## **Clinical Pearl / High-Yield Fact**
In medical statistics, understanding the mean, standard deviation, and variance is crucial for interpreting data, such as drug efficacy, patient outcomes, and laboratory results. Remember, variance (σ^2) equals standard deviation (σ) squared.

## **Correct Answer:** C. 0.0625.