## Core Concept
Mental retardation, now more appropriately referred to as intellectual disability, is classified based on IQ scores. The classification system helps in understanding the level of intellectual functioning and the support needs of an individual. The IQ score ranges are generally categorized as follows: mild, moderate, severe, and profound intellectual disability.

## Why the Correct Answer is Right
The classification of intellectual disability based on IQ scores is as follows:
- **Mild Intellectual Disability:** IQ levels 50-69
- **Moderate Intellectual Disability:** IQ levels 35-49
- **Severe Intellectual Disability:** IQ levels 20-34
- **Profound Intellectual Disability:** IQ levels below 20

Given that an IQ of 15 falls below 20, it corresponds to **Profound Intellectual Disability**. This level of intellectual disability is characterized by significant limitations in intellectual functioning and adaptive behaviors, requiring full support.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because an IQ of 15 does not fall within the range of 50-69, which is considered mild intellectual disability.
- **Option B:** This option is incorrect because an IQ of 15 is not within the range of 35-49, which corresponds to moderate intellectual disability.
- **Option C:** This option is incorrect because, although an IQ of 15 is close to the range of 20-34, it actually falls below this range.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the classification of intellectual disability is not solely based on IQ scores but also on adaptive behaviors. However, for the purpose of many exams and quick references, IQ scores provide a straightforward classification system. It's also important to note that the terms "mental retardation" are outdated and replaced by "intellectual disability."

## Correct Answer: D. Profound.