Answer: scorning the base degrees / By which he did ascend"
Step-by-step explanation:
The piece of evidence best supports the theme that power can corrupt people is "scorning the base degrees / By which he did ascend".
When people attain power, most tend to forget their root and the people who helped them when they didn't have the power. They tend to become proud and not as humble as they were before. This is illustrated in the passage as scorning the base degrees by which he ascend.
The correct option is B.