The correct option is C curse
The word 'boon' refers to something that is helpful or beneficial.
Option A, misfortune refers to an unfortunate condition or event.
Option B, blemish refers to spoiling the appearance or quality of something.
Option C, curse refers to using offensive words in anger or annoyance.
Option D, trouble refers to difficulty or problems.
Thus, according to the meaning, the word 'curse' is the opposite of the word 'boon'.