Before giving the answer, I would like to give the definition of what an ellipsis is. It is a series of dots (typically three, such as "…") that usually indicates an intentional omission of a word, sentence, or whole section from a text without altering its original meaning.
The sentence that uses the ellipses incorrectly is:
D) Researchers reported that they are "...working on devices to assist the
The reason for choosing this sentence is quite simple, it breaks the most important rule of the use of ellipsis: never alter the original meaning of the text or idea.
Besides, by ommiting the word elderly, the sentence loses one of its three main components, the Object ( the elderly), it is indirect object in this case.
