The original sentence intends to compare the hovering of hummingbirds with the hovering of insects such as flies etc. Now if the hummingbirds hover as insects then the literal meaning would be that hummingbirds should change their shape, size and become insects and then hover.
However, the intended meaning is kept when we use "like" instead of "as". The choice is between B and D. D is incorrect because of the use of "that", which refers to the insects instead of the hummingbirds. Additionally, the correct idiomatic form is violated in this answer choice- "One flower to others".
B is the correct choice. Coming on to the question posted in one of the replies about comma before like- ,like insects- The usage of comma is incorrect in such a sentence. The reason is that "like" is used for comparison of nouns . Moreover, for any word which is used to compare two entities, a modifier generally precedes it. In this case the modifier precedes "like". If we put a comma after the word hover(modifier), then
a) like will come after a modifier(verb) in this case. Generally modifiers require a noun close to it for modification. Like is not a noun. It is a preposition. This usage will result in a sentence structure error. A preposition will be placed as a modifier and in this case a modifier will precede a comma followed by a preposition which will be placed as a modifier in the sentence.
b) meaning will change completely. The original sentence with an imaginative comma is given below
More than one hundred years ago, students of ornithology reported that hummingbirds can hover, like insects flitting gracefully from one flower to another.
Hummingbirds can hover but with whom are going to compare their hovering? If you dissect Students of ornithology reported that hummingbirds can hover " - It is a complete thought in itself, so it is an independent clause. We know that according to students of ornithology hummingbirds have a unique quality that they can hover.
"Like insects flitting gracefully from one flower to another" - It is a dependent clause but the two clauses do not seem to be connected. The first clause talks about the hummingbirds and the second talks about insects. Therefore, the meaning is changed completely.