Final answer:
The line from Roosevelt’s speech suggesting journalists should maintain balance in their reporting is option C, which states that those fixated only on the negative can become more harmful than helpful.
Step-by-step explanation:
In President Theodore Roosevelt's speech addressing muckrakers, the line that best supports the main idea that journalists should write honestly without solely focusing on corruption is option C: "...who never thinks or speaks or writes, save of his feats with the muck-rake, speedily becomes, not a help to society, not an incitement to good, but one of the most potent forces for evil." This line encapsulates Roosevelt's view that while exposing corruption is necessary, an obsession with only the negative aspects can turn one into a negative force rather than a champion of progress.