Answer: The correct answer for the blank is -
a. internal fertilization.
Birds perform reproduction by means of internal fertilization, which means that bird egg is fertilized inside the of female by the male sperms.
Birds possess cloaca (a single exit/entrance like repltiles) for entrance of sperm, and release of fertilized egg as well as waste material.
The male fertilizes egg by bringing his sperm at the female cloaca. The female then releases the fertilized egg outside through cloaca so as to allow the further development.
Thus, option a) is the right answer.