**Core Concept**
Mild mental retardation, now more appropriately referred to as intellectual disability, is characterized by significant limitations in both intellectual functioning and adaptive behavior, which covers a range of everyday social and practical skills. This diagnosis is typically made based on IQ scores, with mild intellectual disability being the least severe form of intellectual disability.

**Why the Correct Answer is Right**
The range of IQ scores for mild intellectual disability is generally considered to be between 50-70. This range indicates significant cognitive impairments, but the individual may still possess some adaptive skills and be able to communicate effectively with support. The diagnosis of mild intellectual disability is made when there is a significant discrepancy between an individual's intellectual functioning and their adaptive behavior.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too low, and IQ scores below 40 are typically associated with severe intellectual disability.

**Option B:** This option is incorrect because it is too broad, and IQ scores can range from approximately 50-70 for mild intellectual disability.

**Option C:** This option is incorrect because it is too high, and IQ scores above 70 are typically associated with average or above-average intelligence.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the IQ score is not the sole criterion for diagnosing intellectual disability; adaptive behavior and functional skills also play a crucial role in this diagnosis.

**Correct Answer: D. 50-70**