## **Core Concept**
The question assesses the understanding of typical language and social development milestones in children. It focuses on the relationship between vocabulary size, mode of communication, and developmental age. Understanding these milestones is crucial for pediatricians and developmental psychologists to assess and support children's growth.

## **Why the Correct Answer is Right**
At a developmental age of 18 months, children typically have a vocabulary of around 4-6 words. However, their primary mode of communication remains non-verbal, relying heavily on gestures, pointing, and facial expressions. This stage is characterized by the beginning of language development, where children start to use a few words to communicate but have not yet developed the ability to form sentences or engage in verbal conversations. This matches the description given in the question.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Without a specific age provided, it's hard to directly refute this option. However, typically, by 12 months, children might say a few words like "mama" or "dada" but don't have a vocabulary of 4-6 words.
- **Option B:** At 24 months (2 years), children usually have a vocabulary of around 50 words and start combining two words together. Their mode of communication starts to shift more towards verbal interactions, making this option less likely.
- **Option D:** At 30 months, children's vocabularies expand significantly, often exceeding 100 words, and they begin to form simple sentences. Their primary mode of communication is more verbal than non-verbal, which doesn't fit the description.

## **Clinical Pearl / High-Yield Fact**
A key developmental milestone to remember is that by 18 months, children typically say a few words (like 4-6 words) and start using gestures to communicate. However, it's also crucial to note that significant variability exists among children, and some might need more time to reach certain milestones. Pediatricians often use the "RED FLAGS" for language development: by 12 months, no gesturing (pointing, waving, etc.); by 18 months, no single words; and by 24 months, no two-word phrases.

**Correct Answer: C. 18 months.**