**Core Concept**
Intellectual Quotient (IQ) is a standardized score used to measure human intelligence. The classification of intellectual disability is based on IQ scores, with significant variations in criteria across different countries.

**Why the Correct Answer is Right**
Severe mental retardation, now more commonly referred to as intellectual disability, is characterized by an IQ score below 20-25. This classification is based on the World Health Organization's (WHO) classification system, which also includes mild (IQ 40-54), moderate (IQ 20-39), and profound (IQ below 20) intellectual disability. The IQ score is determined by standardized tests, such as the Wechsler Adult Intelligence Scale (WAIS) or the Wechsler Intelligence Scale for Children (WISC).

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because IQ scores below 20 are classified as profound intellectual disability, not severe.
**Option B:** Incorrect because IQ scores above 40 do not meet the criteria for severe intellectual disability.
**Option C:** Incorrect because this option is incomplete and does not provide a specific IQ score or classification.

**Clinical Pearl / High-Yield Fact**
The American Association on Intellectual and Developmental Disabilities (AAIDD) defines intellectual disability as a significant limitation in intellectual functioning (IQ below 70) and adaptive behavior, which affects an individual's ability to function in daily life.

**Correct Answer:** D.