The correct answer is A. conscientious.
Conscientious means being careful and thorough in your work. It is a synonym for diligent, which is the word used in the question.
B. obedient means following orders or rules. This is not necessarily the same as being conscientious, as someone can be obedient without being careful or thorough.
C. meticulous means paying great attention to detail. This is also not necessarily the same as being conscientious, as someone can be meticulous without being careful or thorough.
D. hard-working means putting in a lot of effort. This is not necessarily the same as being conscientious, as someone can be hard-working without being careful or thorough.
In conclusion, the correct answer is A. conscientious.