Which strategy would a historian most likely use to determine whether a source is valid? a. confirming that it is not an online source b. checking its conclusions against other sources c. determining whether or not it is well-known d. determining whether it is a secondary source

"confirming that it is not an online source" is wrong. The problem is not the fact the source is online, but if it's credible.

"determining whether or not it is well-known" well-known sources might still be wrong. Being known does not make something true.

"d. determining whether it is a secondary source" again, it has nothing at all to do with the validity of the source.

So the answer is:

b. checking its conclusions against other sources

The best way is to check the source and compare to different ones that talk about the same subject. Comparing different conclusions of different authors and seeing different points of view about the same thing is often the best way.