**Core Concept:**
The crown root ratio is a measure of the relationship between the length of the coronal portion of a tooth (crown) and the length of its root. In primary dentition, the root is shorter than in permanent dentition due to the tooth being replaced by an adult tooth later.

**Why the Correct Answer is Right:**
Crown root ratio in primary dentition is approximately 1:1. This means that the coronal portion (crown) is as long as the root. This is due to the short root in primary teeth, which is a normal developmental feature as these teeth are replaced by permanent teeth later in life.

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because the crown root ratio in primary dentition is approximately 1:1, not 2:1.
B. This option is incorrect because the crown root ratio in primary dentition is approximately 1:1, not 3:1.
C. This option is incorrect as the crown root ratio in primary dentition is approximately 1:1, not 4:1.
D. This option is incorrect as the crown root ratio in primary dentition is approximately 1:1, not 5:1.

**Clinical Pearl / High-Yield Fact:**
The primary teeth are essential for normal tooth development, guiding the eruption and alignment of the permanent teeth. Understanding the anatomic features of primary teeth, like the short root, is crucial for dental professionals to diagnose and manage dental anomalies effectively.

**Correct Answer:** D. 1:1 (correct crown-root ratio in primary dentition)