**Core Concept**
The question is testing the concept of Intelligence Quotient (IQ) and its calculation, particularly the definition of mental age and its relevance to intellectual disability. Mental age is a measure of an individual's cognitive abilities compared to their chronological age.

**Why the Correct Answer is Right**
The child's mental age of 4 indicates that their cognitive abilities are equivalent to those of a typical 4-year-old child. This suggests that the child has an intellectual disability, as their mental age is significantly below their chronological age of 10. This is calculated using the formula: IQ = (Mental Age / Chronological Age) x 100. In this case, the child's IQ would be (4 / 10) x 100 = 40.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not provide a clear definition of the child's intellectual abilities or relationship to their chronological age.
* **Option B:** This option is incorrect because it does not accurately describe the child's mental age and its implications for their cognitive development.
* **Option C:** This option is incorrect because it does not provide a relevant or accurate description of the child's intellectual abilities or their relationship to their chronological age.

**Clinical Pearl / High-Yield Fact**
Intellectual disability is a significant cognitive impairment that affects an individual's adaptive behavior and intellectual functioning. It is characterized by a low IQ score, typically below 70, and is often associated with difficulties in learning, communication, and social interactions.

**Correct Answer: a. This child has an intellectual disability.**