I've got a Fast TM that is burning a hole in my pocket. What should I do with it?

If you have, let's say, 5 or less fast TMs, choose D. Otherwise it won't harm to spend it on A or C.

A if you are a frequent solo Machamp raider.
C if you often fight gyms.

A - no. Confusion does a little more damage per second than Psycho Cut. Only worth it if you like dodging.

B,C - Either is good. To fight raid bosses well, it helps to have 2-3 of the optimal counter, so more Rhydons or Machamps with the best moveset is a good thing. Which one to do depends on which raids you want to focus on.

Right now I'm putting dust into a mid level Rhydon (27 now) with ground moves, to get ready to fight Raikou.

What are the ivs of the machamp and rhydon?

I'd go with A. I prefer PC over Confusion because you almost need to dogde with Alakazam's low HP. And B + C require a lot of dust to be useful, and since dust is precious, I'd go with making the higher level mon more useful attacking (imo).

B or C, you need 6 for raid boss.