Assertion $-A$: Species is the basic unit of classification.
Reason $-R$: Species do not change their characteristics over time.

• A
  Assertion $-A$ and Reason $-R$ are both true and Reason $-R$ is the correct explanation for Assertion $-A$.
• B
  Assertion $-A$ and Reason $-R$ are both true but Reason $-R$ is not the correct explanation for Assertion $-A$.
• C
  Assertion $-A$ is true,but Reason $-R$ is false.
• D
  Assertion $-A$ and Reason $-R$ are both false.