I wondered often what my master saw to admire so much in the 'sullen' boy, who never, to my recollection, repaid his indulgence by any sign of gratitude.

In this sentence from the from the passage im not going to read, what does 'sullen' mean?

A. impressive, proud
B. small-framed, slight
C. joyous, appreciative
D. gloomy, resentfully silent