It was time for us ___________ preparation to leave.

Option d) “to start making” is the best choice.

 The sentence says it’s time for us to begin or “start” doing something. That something is “making preparation to leave.” So, “to start making” fits perfectly in the blank and makes the sentence sound right.

The other options don’t fit as well because they don’t sound as good in the sentence, or they don’t follow the rules of how we usually use words in English.