## Core Concept
The vocabulary of a 1.5-year-old child is typically assessed based on language development milestones. At this age, children usually have a limited vocabulary but are beginning to combine words.

## Why the Correct Answer is Right
At 1.5 years old, children usually have a vocabulary of around 5-20 words and start combining two words together. This period is crucial for language development, and most children at this age can say around 5-20 words and may start forming simple phrases.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because it does not accurately reflect the typical vocabulary range for a 1.5-year-old child.
* **Option B:** This option is incorrect because it suggests a vocabulary range that is too low for a 1.5-year-old child.
* **Option D:** This option is incorrect because it suggests a vocabulary range that is too high for a 1.5-year-old child.

## Clinical Pearl / High-Yield Fact
A key point to remember is that by 18 months, children typically have a vocabulary of around 5-20 words and begin to put two words together. Language development can vary, but significant delays should be evaluated.

**Correct Answer: C. 20 words.**