**Core Concept:**
Deciduous teeth, also known as baby teeth, have a unique structure compared to permanent teeth. The outermost layer of a tooth is called the enamel, and its thickness plays a role in the overall tooth structure and function. Prismless enamel refers to the absence of enamel prisms, which are microscopic structures within the enamel, providing strength and resilience.

**Why the Correct Answer is Right:**
The correct answer is D, **1.5 mm**. The thickness of prismless enamel in deciduous teeth is significantly thinner than in permanent teeth. This thinner layer is an essential adaptation for the function of deciduous teeth, which are primarily involved in mastication (chewing) during the early stages of life when the teeth are in use for a shorter duration compared to permanent teeth. The thinner enamel allows for a lighter tooth structure, reducing the overall weight and simplifying the eruption process during tooth replacement by permanent successors.

**Why Each Wrong Option is Incorrect:**
A: **0.9 mm** is too thin and may not provide adequate protection and durability for deciduous teeth.
B: **1.8 mm** is too thick, which would increase tooth weight and may cause difficulties during tooth eruption and replacement.
C: **2.0 mm** is also too thick, providing unnecessary bulk that could lead to unfavorable eruption and replacement.

**Clinical Pearl:**
Understanding the unique features of deciduous teeth, such as their prismless enamel, is crucial for recognizing dental anomalies and guiding dental treatment decisions. Knowledge of tooth anatomy aids in diagnosing and managing various dental issues, including malocclusion, caries, and dental trauma.