The correct answer is B. Sarcastic.
Mordant is an adjective that means “bitterly harsh or satirical.” It can also mean “causing or capable of causing great pain or distress.”
Sarcastic is an adjective that means “using or characterized
by irony, especially in order to mock or convey contempt.”Mild is an adjective that means “not severe or intense.”
Festering is an adjective that means “developing pus or other matter as a result of infection.”
Depressed is an adjective that means “in a state of low spirits, typically one of gloom, despondency, or sadness.”
In the context of the question, the word “mordant” is used to describe a person’s tone of voice. The person’s tone of voice is sarcastic, which means that they are using irony to mock or convey contempt. The other options are not synonyms of “mordant.”