The correct option is A optimistic
The word 'cynical' refers to being doubtful as to whether something will happen or whether it is worthwhile.
Option A, mature refers to being fully developed and grown.
Option B, optimistic refers to being hopeful and confident about the future.
Option C, eccentric refers to being unconventional and slightly strange.
Option D, crazy refers to being extremely enthusiastic.
Thus, according to the meaning, the word 'optimistic' is the opposite of the word 'cynical'.