Change the voice: They will keep all banks open this Sunday.

All banks will be kept open this Sunday by them.

All banks will be keeped open this Sunday by them.

This Sunday all banks opened by them.

None of these

Active voice: They will keep all banks open this Sunday.

Passive voice structure (Future Simple):

will be + past participle

keep → kept (past participle)

Correct passive: All banks will be kept open this Sunday by them.