**Core Concept**
Mental retardation, now referred to as intellectual disability, is a condition characterized by significant limitations in both intellectual functioning and in adaptive behavior, which covers a range of everyday social and practical skills. The intelligence quotient (IQ) is a score derived from standardized tests designed to measure human intelligence, and it is used to categorize intellectual disability into different levels of severity.

**Why the Correct Answer is Right**
The classification of intellectual disability is based on IQ scores, with four main levels: mild, moderate, severe, and profound. An IQ of 15 falls under the severe category, indicating significant limitations in intellectual functioning and adaptive behavior. Individuals with severe intellectual disability may require significant support in daily life and may have difficulty with communication, self-care, and social interactions.

**Why Each Wrong Option is Incorrect**
* **Option A:** Mild intellectual disability is typically characterized by an IQ between 50-69, which is not the case for an IQ of 15.
* **Option B:** Moderate intellectual disability is typically characterized by an IQ between 35-49, which is still higher than an IQ of 15.
* **Option C:** Profound intellectual disability is typically characterized by an IQ below 20, which is more severe than an IQ of 15.

**Clinical Pearl / High-Yield Fact**
Keep in mind that the classification of intellectual disability is not solely based on IQ scores, but also takes into account the individual's adaptive behavior and functional abilities.

**Correct Answer:** D. Severe.