This question is a bit tricky, but I believe it is the answer on the bottom left. The other answers don't make quite as much sense as that one.
"Chocolate is a now sweet treat that is very popular in the United States."
It implies that chocolate wasn't always a sweet treat in the first section of the question, but if you pay close attention; it says that it "always hasn't been" a sweet treat, that use of word choice would mean it is now a sweet treat.
That's why I'd choose the one on the bottom left.