Answer:
C
Step-by-step explanation:
take a look at this line again
performing breathtaking feats
of aquatic acrobatics for the sheer joy of it.
Does that sound like silly and foolish to you? It doesn't to me. That answer should never be considered. Not D
Where's the stubborn part? Do the dolphins sound stubborn? Not B either.
What about flexible and powerful? I think this is likely your second best answer. It would take power to get them out of the water and to be acrobatic you need to be flexible. I wouldn't pick this answer, but I understand those who would.
To me the answer if C. They sound like they are having a lot of fun. And they are treating what they are doing with a lot of glee and much joy. What the author says is that it is breathtaking. C is the only answer you could pick.