**Core Concept**
Mental retardation, now referred to as intellectual disability, is a condition characterized by significant impairments in cognitive functioning and adaptive behaviors. The diagnosis is primarily based on intelligence quotient (IQ) scores and the presence of significant limitations in two or more adaptive behaviors that affect everyday, general living.

**Why the Correct Answer is Right**
The correct answer is based on the current diagnostic criteria established by the American Psychiatric Association (APA). According to the DSM-5, a person is considered to have intellectual disability when their IQ score falls below 70, and they also exhibit significant limitations in adaptive behaviors. This cutoff is chosen because it represents a significant deviation from the average population IQ score, which is around 100. Moreover, the World Health Organization (WHO) defines intellectual disability as a condition with an IQ score below 70, emphasizing the importance of this threshold in clinical practice.

**Why Each Wrong Option is Incorrect**
**Option A:** IQ scores below 100 are considered below average, but they do not necessarily indicate intellectual disability. Many people with IQ scores in this range lead normal, productive lives with no significant cognitive impairments.
**Option B:** An IQ score below 90 is generally considered to be at the lower end of the normal range, and it is not typically associated with intellectual disability.
**Option C:** An IQ score below 80 may indicate some cognitive impairments, but it is not the threshold used to diagnose intellectual disability.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the diagnosis of intellectual disability involves not only IQ scores but also the presence of significant limitations in adaptive behaviors. These limitations can affect various aspects of daily life, such as communication, self-care, and social skills.

**✓ Correct Answer: D. <70**