Compressed Natural Gas (CNG) is mainly composed of

1. A

   Methane

   Option (a) claims that Compressed Natural Gas is mainly composed of methane. This option correctly identifies methane as the principal constituent, accounting for about 85-90 percent of its total volume. Thus, this option represents the correct scientific fact.

2. B

   Propane

   Option (b) claims that propane is the main component of Compressed Natural Gas. While propane can be present in trace amounts or in liquefied petroleum gas, it is not the primary constituent of CNG. Therefore, this option is incorrect.

3. C

   Ethane

   Option (c) claims that ethane makes up the primary composition of Compressed Natural Gas. Although ethane is present in minor quantities alongside other higher hydrocarbons, methane remains the dominant gas by a wide margin. Hence, this option is incorrect.

4. D

   Butane

   Option (d) claims that butane is the primary component of Compressed Natural Gas. Butane is typically the main component of commercial LPG rather than CNG, making this option factually wrong.

Summary. Official key is (a) because Methane constitutes the overwhelming majority of Compressed Natural Gas. Option (b) names propane, which is characteristic of LPG rather than natural gas. Option (c) and option (d) mention ethane and butane respectively, which are minor constituents or found in other petroleum fractions. Understanding the exact chemical makeup is essential for distinguishing between various fossil fuel gases. Thus, only option (a) provides the accurate composition.