Answer:
Unfortunately.
Explanation:
In the sentence, the comma should be put next to 'unfortunately'. That is because, the rest of the sentence can stand by itself, meaning the rest of the words in the sentence excluding "unfortunately" is an independent clause. They make a meaningful and complete sentence too. Also,"unfortunately" is a dependent clause that starts the sentence. So, the use of the comma after it is a mandatory rule.
So, the final sentence will look like this-
Unfortunately, we had not brought our reusable water bottles and had to buy bottled water.
