The correct option is C important
The word 'trivial' refers to something of little value or importance.
Option A, difficult refers to needing much effort or skill to accomplish.
Option B, easy refers to achieving without great effort.
Option C, small refers to something insignificant or unimportant.
Option D, important refers to something of great significance or value.
Thus, according to the meaning, the word 'important' is the opposite of the word 'trivial'.