The correct option is
B Dissatisfied
The word 'smug' refers to having or showing an excessive pride in oneself or one's achievements.
Option A, satisfied refers to something fulfilling or pleasing.
Option B, dissatisfied refers to being not happy or content with something.
Option C, serious refers to acting or speaking sincerely and in earnest rather than joking.
Option D, delighted refers to feeling or showing great pleasure.
Thus, according to the meaning, the word 'dissatisfied' is the most opposite in meaning from the word 'smug'.