## Core Concept
The development of deciduous teeth, also known as primary teeth, begins early in embryonic life. This process involves the interaction of oral ectoderm with the underlying mesenchyme, leading to the formation of dental laminae, which are thickenings of the oral ectoderm.

## Why the Correct Answer is Right
The first sign of future deciduous teeth appears as a thickening of the oral ectoderm, forming the dental lamina. This initial developmental process starts around the sixth week of intrauterine life. The dental lamina gives rise to the development of the enamel organs, which eventually form the teeth. Therefore, the correct timeline for this initial appearance is about the sixth week.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests an earlier timeline than the correct answer. While early embryonic development is a continuous process, the specific formation of dental lamina for deciduous teeth starts later than this.
- **Option B:** This option suggests a later timeline. Although development continues and specific tooth buds form later, the initial thickening appears earlier than this.
- **Option D:** This option also suggests a later timeline than the correct answer. The initial sign of deciduous teeth development is indeed before this period.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the development of teeth starts early, with the first deciduous teeth (central incisors) beginning to calcify around the 3-4 month mark. However, the initial sign of their development as a thickening of the oral ectoderm appears much earlier, around the sixth week of gestation.

## Correct Answer: C. sixth week.