**Core Concept**
The problem involves understanding the properties of a **normal distribution**, where the mean, median, and mode are equal. In this case, the mean hemoglobin level is 13.5 gm%.

**Why the Correct Answer is Right**
Since the distribution is normal, the mean (13.5 gm%) divides the population into two equal halves. Therefore, exactly half of the population will have a hemoglobin level above 13.5 gm%, and the other half will have a level below 13.5 gm%.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it suggests a proportion less than half of the population.
**Option B:** This option is incorrect because it suggests a proportion less than half of the population.
**Option C:** This option is incorrect because it suggests a proportion less than half of the population.

**Clinical Pearl / High-Yield Fact**
In a normal distribution, the mean is a useful measure of central tendency, dividing the population into two equal parts.

**Correct Answer:** D. 0.5