The correct answer is D) increasing the number of keywords and using the word "or."
If an Internet search on a research topic fails to generate enough results, the strategy that is most likely to increase the number of hits is increasing the number of keywords and using the word "or."
This way you are increasing the possibility of better results because the searching engine has more options to look for. Yo have some good options to improve your search. For instance, you can use more specific terms or very unique for what you are looking for. You also can use quotation marks for exact words or phrases, and you can use capital letters to highlight some words.