**Core Concept**
Mental retardation, now more appropriately termed intellectual disability, is a condition characterized by significant limitations in both intellectual functioning and adaptive behavior, which covers a range of everyday social and practical skills. Intellectual functioning is typically measured using intelligence quotient (IQ) scores, which assess cognitive abilities such as reasoning, problem-solving, and learning.

**Why the Correct Answer is Right**
Intellectual disability is categorized based on IQ scores. The American Association on Intellectual and Developmental Disabilities (AAIDD) and the American Psychiatric Association (APA) use the following classification system:
- Mild intellectual disability: IQ 51-70
- Moderate intellectual disability: IQ 36-50
- Severe intellectual disability: IQ 20-35
- Profound intellectual disability: IQ below 20
Individuals with an IQ of 51-70 are considered to have mild intellectual disability, as they may exhibit some limitations in cognitive and adaptive skills but are generally able to learn practical skills and function in everyday life with some support.

**Why Each Wrong Option is Incorrect**
**Option A:** This option does not specify a valid classification of intellectual disability.
**Option B:** Moderate intellectual disability would correspond to an IQ of 36-50, not 51-70.
**Option C:** Severe intellectual disability would correspond to an IQ of 20-35, not 51-70.
**Option D:** Profound intellectual disability would correspond to an IQ below 20, not 51-70.

**Clinical Pearl / High-Yield Fact**
When assessing intellectual disability, it's essential to consider not only the IQ score but also the individual's adaptive behavior and functional skills, as these can vary significantly among individuals with the same IQ level.

**Correct Answer: B. Moderate intellectual disability.