The original question is:
There was an ________ tone to that final message, as if the Speaker found it amusing; and Jonas had smiled a little, though he knew what a grim statement it had been.
Answer:
ironic
Explanation:
The irony in a text happens when a character says one thing, which actually means another. This can be seen in the excerpt above, where the speaker delivered a message as amusing, but it was a dark and unsettling message, but he used irony to speak in a fun and animated way.
