When searching for a source to use in a research paper, the source should be?

reliable

credible

relevant

all of the above

They should be all of the above. There are different was to determine if they are reliable and credible (which closely mean the same thing just slightly different). Reliable means you can count on the source to be accurate. Credible means you can count on the source to be factual. And relevant means the source is current (has the most current information possible).