Which action and action type should I use for withdrawing a resignation?

There's no predefined action available for withdrawing a resignation. You need to create a new action using the Configure Actions task in the Workforce Structures work area.

You need to then associate the action with the Reverse Termination action type (Code: ORA_EMPL_REV_TERMINATION) and the Voluntary termination type. The newly created action will then appear in the Actions list in the Withdraw Resignation page.