Final answer:
The CEO of the entity being audited generally signs the legal letter.
Step-by-step explanation:
In general, the CEO of the entity being audited is the one who signs the legal letter.
Although the board of directors is an important institution of corporate governance, they do not usually sign legal letters. The audit partner, on the other hand, is responsible for reviewing the company's financial records but may not have the authority to sign legal letters. While the entity's attorneys may provide legal advice, they do not typically sign legal letters on behalf of the entity.
Therefore, the CEO of the entity being audited is the most likely individual to sign the legal letter.