$A$: All birds,except the one like Koel (cuckoo),build nests for retiring and taking rest during night time.
$R$: Koel lays eggs in the nests of tailor bird.

• A
  Assertion and Reason both are correct and Reason is the correct explanation of Assertion.
• B
  Assertion and Reason both are correct but Reason is not the correct explanation of Assertion.
• C
  Assertion is correct,but Reason is incorrect.
• D
  Both Assertion and Reason are incorrect.