The correct option is D pleasant
The word 'dull' refers to something lacking interest and excitement.
Option A, serious refers to acting and speaking in earnest.
Option B, wasteful refers to using or spending something carelessly or extravagantly.
Option C, bluffing refers to trying to deceive someone.
Option D, pleasant refers to giving a sense of happy satisfaction or enjoyment.
Thus, according to the meaning, the word 'pleasant' is the opposite of the word 'dull'.