The correct option is B inadequate
The word 'sufficient' refers to being enough or adequate.
Option A, short refers to something relatively small in extent.
Option B, inadequate refers to something not enough or insufficient for a purpose.
Option C, small refers to something insignificant or unimportant.
Option D, more refers to something of a greater extent.
Thus, according to the meaning, the word 'inadequate' is the opposite of the word 'sufficient'.