The Merge Contacts only has one option which allows you to be able to swap the ID numbers when merging contacts.
NOTE: Infusionsoft does not allow you to change the ID numbers when merging Contacts. Although I do not why this is not allowed, because it does not matter when merging contacts via the API. Swapping the ID numbers has no affect to the merging of the Contacts. If you in doubt, then trying merging some dummy contacts first.