## **Core Concept**
The problem involves understanding the properties of a normal distribution, specifically in the context of hemoglobin levels in a population. In a normal distribution, the mean, median, and mode are all equal. The normal distribution is symmetric around the mean, which divides the distribution in half.

## **Why the Correct Answer is Right**
Given that the mean hemoglobin level is 13.5 gm%, and knowing that the normal distribution is symmetric around the mean, exactly half of the population will have hemoglobin levels above 13.5 gm%, and the other half will have levels below 13.5 gm%. This symmetry is a fundamental property of the normal distribution.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests a specific proportion but without calculation or context, it's hard to directly refute. However, given the symmetry of the normal distribution, any option suggesting a proportion other than 50% would be incorrect for the portion above the mean.
- **Option B:** Similar to Option A, without specific calculation, it's implied to be incorrect because it does not represent half of the population.
- **Option D:** This option also does not represent half of the population and is therefore incorrect based on the principles of a normal distribution.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in a normal distribution, the mean divides the distribution in half. Therefore, without needing to calculate z-scores or percentiles, one can infer that 50% of the population will have values above the mean, and 50% will have values below.

## **Correct Answer:** C.