Final answer:
The phrase 'Then wherefore sully the entrusted gem' is best understood as a metaphor for damaging something valuable that has been entrusted to someone.
Step-by-step explanation:
The quote "Then wherefore sully the entrusted gem" can be interpreted as B. A metaphor for damaging a valuable item. When analyzed within the context of the texts provided, this line appears to imply the act of tarnishing or compromising something that is precious and has been entrusted to someone. This concept links closely to the scenarios described where items or information of great value and sensitivity are mishandled or misused. Whether it is a physical object like a tooth in McTeague's world or the more abstract concept of confidential information in the case of Dupin, the metaphor underscores the importance of safeguarding what is valuable and the consequences of failing to do so.