Answer:
C
Explanation:
A. The weather was so hot that many people wore shorts every day.
^ this is clearly a fact. the person is observing that, for multiple days, people wore shorts because of the heat.
---------
B. My brother and I went to three different water parks this summer.
^ this is clearly a fact. the person is saying that they and their brother went to multiple water parks over the summer season.
---------
<h2>
<u><em>C. We will remember this as the best summer of our lives.</em></u></h2>
^ this is an <u><em>opinion</em></u>. someone else might think it was horrible, or just like any other summer. someone else may not like water parks at all, and to them this would have been a terrible, or rather an ''eh'' summer.
---------
D. Dad saved up enough money to buy a boat for the lake.
^ this is clearly a fact. the person is saying that their dad had been saving up money, and finally had saved up enough to buy a boat that summer.