I will dissect each sentence for you.
A. Do you think the hummingbirds will come back!
No, this isn't it. It's a question, so it needs to end in a ?.
B. Hummingbirds are kind of interesting!
This might be it, considering the fact that this a direct sentence, but the exclamation point isn't needed.
C. When I see a hummingbird, I get so excited?
No, this isn't a question.
D. I love hummingbirds!
Maybe, since this is more of stating a fact and getting excited about the hummingbird which is nonetheless an animal/object.
Left choices:
B and D.
I'd have to say...
D, since B is kind of just stating something is interesting rather than loving something, however, both are punctuated correctly.
Answer:
D