## Core Concept
The question tests knowledge of pediatric dentition, specifically the timeline for the eruption of primary (milk) teeth. Primary teeth, also known as baby teeth or milk teeth, begin to erupt when a child is around 6 months old and continue to erupt until they are about 3 years old.

## Why the Correct Answer is Right
The correct timeline for the eruption of all primary teeth is by the age of 3 years. This process starts with the eruption of the lower central incisors around 6 months of age and continues until the second molars erupt, usually by the time the child is 2-3 years old. The sequence and timing can vary slightly among children, but generally, by 3 years of age, all 20 primary teeth have erupted.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incomplete and cannot be evaluated.
- **Option B:** This option suggests a timeline that is too early for all primary teeth to have erupted.
- **Option D:** This option suggests a timeline that is too late for the completion of primary teeth eruption.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the eruption of primary teeth follows a somewhat predictable sequence and timeline. Typically, the first teeth to erupt are the lower central incisors, followed by the upper central incisors, and then the process continues with the eruption of lateral incisors, canines, and molars. By 3 years of age, all primary teeth should have erupted, making this a critical milestone in pediatric dentistry.

## Correct Answer: C. 3.