**Core Concept**
Porcelain is a type of ceramic material known for its translucency, strength, and resistance to chemical corrosion. It is primarily composed of a specific combination of inorganic compounds that provide its unique properties.

**Why the Correct Answer is Right**
The main constituent of porcelain is **silicon dioxide (SiO2)**, also known as silica. Silica is a glassy, amorphous form of silicon dioxide that is abundant in nature and is often found in sand, quartz, and other minerals. When heated to high temperatures, silica undergoes a process called vitrification, where it transforms into a glassy, non-crystalline structure that provides porcelain with its characteristic hardness and durability. The presence of silica in porcelain also contributes to its translucency and resistance to chemical corrosion.

**Why Each Wrong Option is Incorrect**
**Option A:** **Aluminum oxide (Al2O3)** is a common constituent of some ceramic materials, but it is not the main constituent of porcelain.
**Option B:** **Calcium carbonate (CaCO3)** is a mineral found in limestone and chalk, but it is not a primary component of porcelain.
**Option C:** **Feldspar** is a type of mineral that is often used as a flux in ceramic production, but it is not the main constituent of porcelain.

**Clinical Pearl / High-Yield Fact**
Porcelain is often used in dental and medical applications due to its biocompatibility and resistance to corrosion. However, it can be prone to thermal shock, which can cause it to crack or shatter when exposed to sudden temperature changes.

**Correct Answer:** C.