Final answer:
The tone of Mary Mallon's letter is apologetic, as she expresses regret and remorse for the harm caused by her actions.
Step-by-step explanation:
The tone of Mary Mallon's letter can best be described as apologetic.
In the letter, Mary expresses regret and remorse for the harm caused by her actions as an asymptomatic carrier of typhoid fever.
She acknowledges her responsibility and apologizes for any pain she may have caused.