## Core Concept
Mental retardation, now more commonly referred to as intellectual disability, is characterized by significant limitations in both intellectual functioning and adaptive behavior. The intellectual functioning is often assessed through IQ (intelligence quotient) tests. A person is generally considered to have intellectual disability if their IQ score is below a certain threshold.

## Why the Correct Answer is Right
The correct answer, , corresponds to an IQ score below 70. This threshold is widely accepted in the field of psychology and psychiatry as the cutoff for intellectual disability. The classification is based on the understanding that an IQ score below 70 indicates significant cognitive impairments. The World Health Organization (WHO) and the American Association on Intellectual and Developmental Disabilities (AAIDD) support this criterion.

## Why Each Wrong Option is Incorrect
* **Option A:** . This option suggests an IQ score below 50 as the criterion for mental retardation, which is too low and does not align with the widely accepted threshold of 70.
* **Option B:** . This option suggests an IQ score below 85, which is not considered indicative of intellectual disability as it falls within one standard deviation of the mean in a normal distribution of IQ scores.
* **Option D:** . This option suggests an IQ score below 90, which is even higher than option B and clearly not indicative of intellectual disability.

## Clinical Pearl / High-Yield Fact
It's crucial to remember that the diagnosis of intellectual disability involves more than just IQ scores; it also requires significant limitations in adaptive behaviors that cover many everyday social and practical skills. This holistic approach ensures that individuals are not misclassified or overlooked.

## Correct Answer: .