Answer:
They spent three days’ vacation building terraces and landscaping the yard!
Step-by-step explanation:
The sentence "they spent three days’ vacation building terraces and landscaping the yard!" has a possessive plural noun. When there is a plural noun that ends with a final "s" the apostrophe must be placed after the "s" of the plural noun. It is not correct to say "They spent three day's vacation" because It is not a day or one day in its singular form. It's been three days.