$A:$ An organism is the basic unit of an ecosystem.
$R:$ The interaction between biological communities forms the structure of ecology.

• 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
  Both $A$ and $R$ are false.