Question

When conducting research online the keyword or Key phrase you type into a search engine should be

Answer 1

If those are the options I would say that the correct one is C  as clearly defined as possible so in order to narrow your search.
The purpose of this is to be as accurate as possible so you can find exactly what you are looking for.  Today, it is possible to use SEO keywords which makes the process easier due to it exactitude within the search. 

Answer 2

As clearly defined as possible so in order to narrow your search