Assertion $-A$: The term 'genus' is a single term.
Reason $-R$: $A$ genus may have one or more species; a species may be a single or a combined term.

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