The correct answer is D.
N/A is a common abbreviation and it stands for not applicable, not available, no answer.
It is used to indicate when certain piece of information is not provided, either because it does not apply to a particular case in question or because the answer is not available.
The usage of N/A dates back to the year 1920.
The N/A abbreviation is used to distinguish a blank space that can not be filled due to lack of information and it is not left blank erroneously.
