## Core Concept
The eruption of temporary teeth, also known as primary teeth, is a significant milestone in a child's dental development. This process involves the emergence of 20 teeth through the gums, which typically starts around 6 months of age and continues until the child is about 3 years old. Understanding the timeline of primary teeth eruption is crucial for pediatric dentistry and overall child health.

## Why the Correct Answer is Right
The correct answer indicates that the eruption of temporary teeth is completed by a specific age. Generally, the first primary tooth erupts around 6 months of age, and the process continues until the child is about 3 years old. By the age of 3, all 20 primary teeth should have erupted. This timeline is essential for monitoring the dental development of children.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests an age that is too early for the completion of primary teeth eruption. If the first tooth erupts around 6 months, it's unlikely all 20 teeth would be out by a much younger age.
- **Option B:** Similarly, this option proposes an age that might be too soon for the eruption process to be complete, given the typical timeline.
- **Option D:** This option suggests an age that is later than necessary for the completion of primary teeth eruption. While some children might develop at slightly different rates, 3 years is generally considered the benchmark.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the sequence and timing of primary teeth eruption can vary among children, but generally, the lower front teeth (central incisors) are the first to erupt, followed by the upper front teeth, and then the process continues with the eruption of molars, canines, and second molars. By 3 years of age, children typically have a full set of 20 primary teeth.

## Correct Answer: C. 3 years.