The correct option is A certain
The word 'sceptical' refers to having doubts or reservations about something.
Option A, sanguine refers to being optimistic even in a bad situation.
Option B, hopeful refers to feeling or inspiring optimism for a future event.
Option C, certain refers to something sure and unquestionable.
Option D, doubtful refers to feeling uncertain about something.
Thus, according to the meaning, the word 'certain' is the opposite of the word 'sceptical'.