<h2>
Answer:</h2>
a. Es buena idea que hagas una reservación.
<h2>
Explanation:</h2>
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 <em>you recommend. </em>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 <em>doubts, desires, unknown things, abstract things, and emotions. </em>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:
<em>It's a good idea for you to make a reservation.</em>
