Foremost :

Answer: Option B

Hindmost : furthest back, latest or ultramodern.
Disposed : inclined or willing.
Mature : fully developed physically, full-grown.
Premature : occurring or done before the usual or proper time, too early.
Unimportant : lacking in importance or significance.
Foremost : most prominent in rank, importance, or position.

So, antonym is Unimportant.