Final answer:
The correct answer is C. obligatory; decide; optional. If you want to take a vacation, it is obligatory to comply with buying a plane ticket. However, whether you go or not is optional depending on your preference.
Step-by-step explanation:
The correct answer to the question is C. obligatory; decide; optional.
In the given statement, it is stated that if you want to take a vacation, then you should buy a plane ticket. It is obligatory to comply with that if you have business on your vacation. However, it is optional to go if you do not want to go.