$A$: Biogas is a mixture of various gases.
$R$: The type of gases produced depends on the microbes and the organic substrate used.

• A
  Both $A$ and $R$ are true and $R$ is the correct explanation of $A$.
• B
  Both $A$ and $R$ are true but $R$ is not the correct explanation of $A$.
• C
  $A$ is true but $R$ is false.
• D
  $A$ is false but $R$ is true.