**Core Concept**
Mental retardation, now more appropriately referred to as Intellectual Disability (ID), is a neurodevelopmental disorder characterized by significant impairments in cognitive and adaptive functioning. The diagnosis of ID is primarily based on an individual's Intelligence Quotient (I.Q.) score, which is a measure of their cognitive abilities compared to the general population.

**Why the Correct Answer is Right**
The World Health Organization (WHO) and the American Association on Intellectual and Developmental Disabilities (AAIDD) define ID as an I.Q. score below 70, with an accompanying significant impairment in adaptive behavior. Adaptive behavior refers to the skills and abilities necessary for everyday life, such as communication, social skills, and self-care. I.Q. scores are calculated based on standardized tests that assess cognitive abilities such as reasoning, problem-solving, and memory.

**Why Each Wrong Option is Incorrect**
**Option A:** Not provided
**Option B:** Not provided
**Option C:** Not provided

**Clinical Pearl / High-Yield Fact**
It's essential to note that I.Q. scores are not the sole determinant of ID. Adaptive behavior and functional limitations play a crucial role in the diagnosis and assessment of ID. Additionally, I.Q. scores should be interpreted in the context of the individual's cultural, linguistic, and socio-economic background.

**Correct Answer:** D.