Step 1: Concept
Biogas is produced by the anaerobic decomposition of organic matter.
Step 2: Meaning
It is a mixture of various gases, but it primarily contains flammable hydrocarbons.
Step 3: Analysis
While biogas contains $CO_{2}$ and small amounts of other gases, its main constituent (usually 50-75%) is Methane ($CH_{4}$).
Step 4: Conclusion
Methane ($CH_{4}$) is the major component of biogas.
Final Answer: (A)