**Core Concept**
Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. Mosaicism is a variant of Down's syndrome where only some cells in the body have the extra chromosome 21. This condition can lead to varying degrees of intellectual disability.

**Why the Correct Answer is Right**
In individuals with Down's syndrome, the presence of an extra copy of chromosome 21 disrupts normal brain development and function. This results in cognitive impairments, which can range from mild to severe. Mosaicism can affect the severity of the cognitive impairment, but it is still a form of Down's syndrome. The IQ range for individuals with Down's syndrome can vary, but it is generally lower than the average IQ of the general population. Research suggests that the IQ range for individuals with Down's syndrome can be between 40-70, with some individuals having higher or lower IQ scores.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify any particular IQ range for Down's syndrome. Down's syndrome is a heterogeneous condition, and IQ scores can vary widely among individuals.
* **Option B:** This option is incorrect because it implies that Down's syndrome is associated with a very high IQ range, which is not consistent with the general understanding of the condition.
* **Option D:** This option is incorrect because it implies that Down's syndrome is associated with a very low IQ range, which is not the case for all individuals with the condition.

**Clinical Pearl / High-Yield Fact**
It's essential for clinicians to remember that individuals with Down's syndrome have a range of cognitive abilities, and their IQ scores can vary. However, most individuals with Down's syndrome will have some degree of intellectual disability.

**Correct Answer: C. 40-70**