**Core Concept**
The question is testing the student's understanding of the normal distribution curve and its statistical significance. In a normal distribution, the area under the curve is measured in terms of standard deviations (S.D.) from the mean, which helps in understanding the probability of a value or a range of values occurring within a given population.

**Why the Correct Answer is Right**
The correct answer is based on the 68-95-99.7 rule, also known as the empirical rule. This rule states that about 95% of the values in a normal distribution curve lie within two standard deviations of the mean. This is because the normal distribution curve is symmetric, and the area under the curve is divided into three parts: 34.1% within one standard deviation, 47.7% within two standard deviations, and 13.6% beyond two standard deviations. Therefore, the area falling under 2 S.D. curve would be around 95%.

**Why Each Wrong Option is Incorrect**
**Option A:** 66% is incorrect because it represents the area within one standard deviation of the mean, not two standard deviations.
**Option C:** 57% is incorrect because it is a rough estimate of the area within one standard deviation of the mean, not two standard deviations.
**Option D:** 99% is incorrect because it represents the area within three standard deviations of the mean, not two standard deviations.

**Clinical Pearl / High-Yield Fact**
It's essential to remember the 68-95-99.7 rule for understanding the probability distribution of various health metrics, such as blood pressure, cholesterol levels, or body mass index, in a population.

**✓ Correct Answer: B. 95%**