Which best describes marcus' tone in this excerpt from little brother? mom gave me a long hug, and i noticed how thin her arms had gotten, how saggy the skin on her neck was. i always thought of my mother as young, pale, rosy-cheeked and cheerful, peering shrewdly through her metal-rim glasses. now she looked a little like an old woman. i had done that to her. the terrorists had done that to her. the department of homeland security had done that to her.

Remorseful.

Explanation:

The novel "Little Brother" by Cory Doctorow tells the story of how four teenagers were wrongfully arrested and accused by the Department of Homeland Security for a terrorist invasion. It details the narrator Marcus's fight against the DHS and the American rights that are being violated by the government agencies against the normal citizens.

Based on the excerpt provided in the question, the narrator seems remorseful for the way his mother had become, all because of him. Marcus and his friends had been detained and accused to be involved in the terrorist attack that destroyed the Bay Bridge. This stress and accusations have made his mother age more faster and is taking a toll on her. When he said that she used to be "young, pale, rosy-cheeked and cheerful, peering shrewdly through her metal-rim glasses". But now, she is "like an old woman" and he "had done that to her". This shows his remorse on making his mother have to go through all these.