**Core Concept**
Child development red flags are early warning signs that indicate potential developmental delays or disorders. These red flags are essential for pediatricians to identify and refer children to specialists for further evaluation. In this context, red flags are specific milestones that children typically achieve by certain ages, and their absence may indicate a need for closer monitoring or intervention.

**Why the Correct Answer is Right**
Children who have not achieved certain milestones by specific ages may be at risk for developmental delays or disorders. Not saying a single word by 18 months (Option A) is a red flag for language delay or disorder. Similarly, not sitting without support by 12 months (Option B) may indicate gross motor delay or disorder. Finally, not walking alone by 18 months in boys (Option C) is a red flag for gross motor delay or disorder, particularly in boys, as they tend to walk later than girls. Therefore, all of these options are red flags in child development.

**Why Each Wrong Option is Incorrect**
**Option A:** While not saying a single word by 18 months is a red flag, it is not the only red flag. Children who are not talking may still be developing other skills.

**Option B:** Not sitting without support by 12 months is a red flag, but it is not the only one. Children who are not sitting may still be developing other skills, such as crawling or standing.

**Option C:** Not walking alone by 18 months in boys is a red flag, but it is not the only one. Children who are not walking may still be developing other skills, such as crawling or standing.

**Clinical Pearl / High-Yield Fact**
Children with developmental delays or disorders often have other red flags, including loss of previously acquired skills, unusual body posture, or difficulty with social interactions. Pediatricians should use a combination of red flags and developmental screening tools to identify children at risk.

**Correct Answer Line**
✓ Correct Answer: D. All of the above