The Scene:
Old work bathroom remodel. GC has it down to the studs.
I'm keeping the existing circuit for lights and fan. 12/2 NM, no grounding
I've pulled a homerun 20a for receps.

How to ground the lights and fan?
A. There's a bare 14ga conductor that was bonding all of the old metal boxes. It is clamped with a grounding electrode to the bathroom's water pipe. This wire then continues off to other parts of the house. I could tap into this. But! Code says bonding to water pipe has to be 5' from source. This seems like a 'better than nothing' option. My least favorite so far.

B. Connect to the grounding of the new 20a circuit. This goes back to the panel and straight to the rod. Problem- I scoured the code and couldn't find an allowance for this.

C. Connect a new electrode and ground the lights and fan to the water pipe.

D. Run 12ga grounding wire all the way back to the panel. This is okay code wise, but raises some new issues.

What's best here?

Since you are down to the studs I would think that a new lighting circuit should be run.

I tend to agree with Jim M.
You may want to run your list of options past the local AHJ, as he is the one to accept or reject your choice(s).

Best practice would be to run a new wire. Your ideas of using the water pipe, even with another rod, won't work unless you bond back to the existing ground electrode system.
You could share the EGC of the new receptacle circuit you have run but you do have to worry about protecting the wire you run. I would ask if your AHJ is OK with just stringing a THHN over to the receptacle box

Guess I'm trying to figure out why anything has to be done with existing. But if you insist on doing something, item D. is the only one that meets code.

I was just trying to let safety trump a technical violation (extending the EGC from an existing circuit of sufficient size). I would rather have a ground than not.

I guess I agree with you Greg and I would add, another option would be to wire the fan and lights in the bathroom on the new receptacle circuit. As long as he didn't extend to anything beyond the one bathroom, he'd have not only grounding but GFCI protection ta boot.

George- I'm liking 210.11(C)3. The lights and the fan sum up to less than 1amp. Plenty of leftover on the 20a dedicated.

Thanks all

The only method that would be questionable would be B but all the others are code compliant