Answer:
C. In a delicately worded letter, Randolph told Washington that no, the president could not move Congress, even in an emergency.
Step-by-step explanation:
This sentence shows the main idea of the passage: exactly what Randolph advised Washington. None of the other sentences work, because the main point is that he said Congress could not be moved.
Test it out: if you remove all the other sentences and leave only C, the main point should still make sense. And it does.