Answer:
a. Es buena idea que hagas una reservación.
Step-by-step explanation:
Option a and c are grammatically correct, but only option a is right in this context because the statement of the problem establishes that we must choose the correct answer for the question about vacations you have taken and what you recommend. Since you have to give a recommendation, you need to say es buena idea que hagas una reservación, because this sentence stands for the subjunctive mood, that is used to express doubts, desires, unknown things, abstract things, and emotions. So the key here is the word haga, which is the conjugation of the verb hacer for the second person singular (tú - informal you) in the subjunctive mood. Finally, this sentence means:
It's a good idea for you to make a reservation.