**Core Concept**
The child's developmental age is being assessed based on their language skills, particularly their vocabulary and mode of communication. This is an example of a developmental milestone, specifically language development, which is a key indicator of a child's cognitive and social abilities.

**Why the Correct Answer is Right**
Children typically develop language skills in a predictable sequence. Around 18-24 months (1.5-2 years), children usually have a vocabulary of around 10-15 words and rely heavily on non-verbal communication. This stage is characterized by the use of single words to communicate, with an increasing ability to combine words to form short sentences. The child's reliance on non-verbal communication, such as gestures and facial expressions, is still prominent at this stage.

**Why Each Wrong Option is Incorrect**
**Option A:** 6-8 months - At this age, children are still in the babbling phase and have a very limited vocabulary, usually around 1-2 words.

**Option B:** 3-4 years - Children at this age typically have a vocabulary of around 100-300 words and use complex sentences to communicate, making option B incorrect.

**Option C:** 2-3 months - At this age, children are still in the cooing phase and have not yet developed any language skills, making option C incorrect.

**Clinical Pearl / High-Yield Fact**
Remember that language development is a continuous process, and children progress at their own pace. However, if a child is not speaking by the age of 2 years, it is considered a red flag and warrants further evaluation by a pediatrician or a speech-language pathologist.

**Correct Answer:** B. 2-3 years.