Answer:
A good counterclaim to the historian's claim would be B. Personal letters written by Julius Caesar indicate that he believed that Rome should always remain a republic.
Explanation:
In the first place let's look at the "historian's claim" Julius Caesar was assassinated because he was secretly plotting to overthrow the roman republic and declare himself emperor. Thus to counterclaim is a statement we have to look for something that shows the opposite.
Then, in second place We have choice A) which uses "historians studies" as an argument to claim that the "historian's claim" is false. But being a studied topic doesn't make it strong enough to counterclaim his perspective.
In third place, We have option B) which in case it existed, knowing that Julius Caesar wrote that, would make us believe he did not wish to become Emperor. So that would be strong enough to counter the historian's claim. Because we would have then a solid proof of Julius Caesar's opinion.
And finally, in fourth place, we have C) Following Julius Caesar's assassination the Rome Republic was replaced by a Roman Empire. Now, this is related to the topic, but it doesn't picture why would Julius Caesar be assassinated, we have nothing related to the origin of assassination.
So the correct answer is b)
Thank you.