## Core Concept
The question pertains to the classification of intellectual disability based on Intelligence Quotient (IQ) scores. Intellectual disability is characterized by significant limitations in both intellectual functioning and adaptive behavior, which covers a range of everyday social and practical skills. The classification of intellectual disability is often categorized based on IQ scores.

## Why the Correct Answer is Right
The correct answer categorizes an IQ score of 20-34. According to the classification of intellectual disability:
- **Mild intellectual disability**: IQ levels 50-69
- **Moderate intellectual disability**: IQ levels 35-49
- **Severe intellectual disability**: IQ levels 20-34
- **Profound intellectual disability**: IQ levels below 20

Therefore, an IQ score of 20-34 falls into the category of **Severe intellectual disability**. This category represents individuals who may have very limited communication skills and require full support in daily life.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not correspond with the classification for an IQ score of 20-34.
- **Option B:** This option suggests a different range or classification that does not match the severe intellectual disability category.
- **Option D:** This option is incorrect as it likely represents another category of intellectual disability that does not align with the IQ range provided.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the classification of intellectual disability is not solely based on IQ scores but also on adaptive behaviors. However, for the purpose of many exams and quick references, IQ scores provide a straightforward categorization. The classification can guide the level of support and intervention required for individuals.

## Correct Answer: C. Severe.