**Core Concept:** Intelligence quotient (IQ) is a measure of an individual's cognitive abilities, usually calculated by taking a standardized test. Mental retardation is defined as a significant impairment in intellectual functioning.

**Why the Correct Answer is Right:** In the field of psychiatry and neurology, mental retardation, also known as intellectual disability, is diagnosed when an individual's IQ is significantly lower than average. IQ is measured on a scale of 0 to 100, with an IQ of 70-75 considered average. Mental retardation is diagnosed when an individual has an IQ below 70.

**Why Each Wrong Option is Incorrect:**

A. Less than 85: This falls within the average range (IQ 70-120), so it does not meet the criteria for mental retardation.
B. Less than 60: While below the average range, this is still considered within the normal range (IQ 50-69).
C. Less than 70: This is within the average range (IQ 70-120), not qualifying for mental retardation.
D. Less than 80: Although below average, this falls within the range of 60-80, not qualifying for mental retardation.

**Why the Correct Answer is Right:** Mental retardation is a clinical condition characterized by significant intellectual disability. A score below 70 on an IQ test indicates a significant impairment in cognitive abilities, meeting the diagnostic criteria for mental retardation.

**Clinical Pearl:** Mental retardation is often associated with comorbidities such as behavioral issues, learning disabilities, and difficulties in daily living skills. Early identification and intervention can significantly improve the individual's quality of life.