## Core Concept
The core concept being tested here relates to child development milestones, specifically language development. At 18 months, children typically experience significant growth in their vocabulary and ability to communicate through words.

## Why the Correct Answer is Right
The correct answer, **20 words**, reflects the average vocabulary size of an 18-month-old child. Research in pediatric development indicates that by 18 months, children usually have a vocabulary of around 20 words and start combining two words together. This milestone is crucial as it signals the beginning of more complex language skills.

## Why Each Wrong Option is Incorrect
- **Option A: 5 words** - This is too low for an 18-month-old child. Typically, by 18 months, children have a more extensive vocabulary than just 5 words.
- **Option B: 50 words** - This is higher than the average expected vocabulary for an 18-month-old. While some children may have more than 20 words, 50 is generally more characteristic of a slightly older child.
- **Option D: 100 words** - This is significantly higher than what is expected for an average 18-month-old child. A vocabulary of around 100 words is more typical for children around 2 years of age.

## Clinical Pearl / High-Yield Fact
A key point to remember is that by 18 months, children should have a vocabulary of around 20 words and should start combining two words together to form simple phrases. Significant delays in language development, such as not having any words by 16 months or not combining two words by 24 months, should prompt a developmental assessment.

## Correct Answer: C. 20 words