## Core Concept
The question assesses the developmental milestones of an 18-month-old infant. At this age, children typically achieve certain physical and cognitive skills. Understanding these milestones is crucial for identifying potential developmental delays.

## Why the Correct Answer is Right
An 18-month-old infant is expected to have certain abilities, including saying a few words, understanding simple commands, pointing to body parts, and possibly walking independently. Option is correct because an 18-month-old can typically say a few words, like "mama" or "dada," and may have a vocabulary of around 5-20 words.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because, although specifics aren't provided, if it suggests a skill beyond the expected range for an 18-month-old, it would be wrong. Typically, an 18-month-old may not be able to have a conversation or use complex sentences.
* **Option B:** Without specifics, if this option implies a skill that is below the expected developmental milestone for an 18-month-old, such as not being able to point or say any words, it would be incorrect.
* **Option D:** This option would be incorrect if it suggests a skill that is not expected for an 18-month-old, such as reading or advanced problem-solving.

## Clinical Pearl / High-Yield Fact
A key developmental milestone for an 18-month-old infant is the ability to say a few words and possibly walk independently. Clinicians often use the "5-word vocabulary" as a rough benchmark; an 18-month-old child is expected to have at least a 5-word vocabulary.

## Correct Answer: D.