**Core Concept**
The question is testing the concept of Intelligence Quotient (IQ) and its calculation, specifically the formula for determining intellectual disability. The Intelligence Quotient (IQ) is a score derived from standardized tests designed to measure human intelligence, typically expressed as a ratio of mental age to chronological age multiplied by 100.

**Why the Correct Answer is Right**
The child's mental age is 4 years, and his chronological age is 10 years. To calculate the IQ, we use the formula: IQ = (mental age / chronological age) x 100. Plugging in the values, we get IQ = (4 / 10) x 100 = 40. This indicates a significant delay in cognitive development, which is characteristic of intellectual disability. The child's low IQ score is likely due to impaired cognitive function, which can be caused by various factors such as genetic disorders, prenatal exposure to toxins, or environmental factors.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided.
* **Option B:** This option is not provided.
* **Option C:** This option is not provided.
* **Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
Intellectual disability is characterized by significant delays in cognitive development, which can manifest as impaired social, adaptive, and communication skills. It is essential to diagnose and manage intellectual disability early to provide appropriate support and interventions to improve the individual's quality of life.

**Correct Answer:** D.