You can assume a. natality is zero as the population is decreasing.
Hope this helps! :)
The correct option is D) immigration
Immigration can be described as the movement of an organism to another place for living. As the scenario in the question tells, the population of the endangered Philippine eagle could be seen only in one place hence we can say that the immigration rate of these organisms was zero because they could not be seen in any other place.
Other option, like option B, cannot be correct because mortality refers to the death rate and obviously many organisms of the species died due to which it became endangered.