## Core Concept
In a normal distribution curve, the mean, median, and mode are all equal. The curve is symmetric about the mean, with the majority of the data points clustered around the mean. The question pertains to understanding the properties of a normal distribution curve.

## Why the Correct Answer is Right
Given that the distribution of hemoglobin levels follows a normal distribution curve and the mean hemoglobin level is 13.5 grams, the curve is symmetric around this mean. This symmetry implies that 50% of the data points lie above the mean, and 50% lie below it. Therefore, the percentage of people having a hemoglobin level more than 13.5 grams is 50%.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests a specific percentage but is incorrect because it does not reflect the understanding that in a normal distribution, 50% of the values are above the mean.
- **Option B:** Similarly, this option is incorrect for the same reason as Option A; it does not accurately represent the proportion of the population above the mean in a normal distribution.
- **Option D:** This option is also incorrect as it implies a different proportion than what is expected in a normal distribution regarding the mean.

## Clinical Pearl / High-Yield Fact
A key point to remember is that in a normal distribution, about 68% of the data falls within one standard deviation of the mean, about 95% falls within two standard deviations, and about 99.7% falls within three standard deviations. However, the question specifically asks about the mean, and the critical fact here is that 50% of the population will have values above the mean, and 50% will have values below it.

**Correct Answer: C. 50%**