For Part A, the best choice that reflects President Roosevelt's opinion of investigative journalists is:
He believed investigative journalists were important, but they should not focus too much on the negative.
For Part B, the sentence that BEST supports this answer is:
"But the man who never does anything else, who never thinks or speaks or writes, save of his feats with the muck rake, speedily becomes, not a help but one of the most potent forces for evil."
This sentence indicates that while Roosevelt acknowledged the importance of exposing social problems, he also warned against an excessive focus on negativity that could lead to a harmful effect.