Which firmware option do I choose for an R6120 router?

> There are 3 firware options to choose from.

   Says who?  What are they?

> We are using [...]

   Should make no difference.  General advice:

   Visit http://netgear.com/support , put in your model number, and look
for Downloads.  (For older versions, under Firmware and Software
Downloads, look for "View Previous Versions".)  Find the kit(s).
Download the kit(s) you want.  Read the "Release Notes" file for
instructions.  (In the User Manual, look for "firmware", and,
especially, for a topic like "Manually Upload Firmware to the Router".)
When that fails because of a deficient User Manual, try:

      https://kb.netgear.com/23960
      https://kb.netgear.com/31573