**Core Concept**
The main constituent gas in Compressed Natural Gas (CNG) is a hydrocarbon that plays a crucial role in fueling vehicles and industrial applications. CNG is a fossil fuel that is composed of various gases, with the primary component being a specific hydrocarbon.

**Why the Correct Answer is Right**
The main constituent gas in CNG is methane (CH4), a simple hydrocarbon molecule composed of one carbon atom bonded to four hydrogen atoms. Methane is the primary component of natural gas, making up approximately 80-90% of its composition. The high energy density and low carbon-to-hydrogen ratio of methane make it an attractive fuel source for vehicles and industrial applications.

**Why Each Wrong Option is Incorrect**
**Option A:** Ethane is a hydrocarbon, but it is not the main constituent gas in CNG. Ethane is a byproduct of natural gas processing and is not as widely used as methane.
**Option B:** Propane is a hydrocarbon used as a fuel, but it is not the primary component of CNG. Propane is commonly used as a cooking fuel and in industrial applications.
**Option C:** Butane is a hydrocarbon with various applications, but it is not the main constituent gas in CNG. Butane is commonly used as a fuel for appliances and in industrial processes.

**Clinical Pearl / High-Yield Fact**
Methane is a potent greenhouse gas, and its release during CNG combustion contributes to climate change. However, the overall carbon footprint of CNG is generally lower than that of gasoline or diesel fuels, making it a more environmentally friendly option.

**Correct Answer: C. Methane**