Hi,
It should be the new C.
In the last 3 or so years it's always been the 772, at least when i have flown, and now given that all their 772s are re-furbished, we always get the new C. I fly this route quite often on TG 319/320 (also, see my username).
I was once chatting on-board with a cabin crew and was told that there are crew restriction on who can fly to KTM. Only pilots/co-pilots who have spent a lot of time training specially for KTM can fly the route, and so it's rare for anything else to be flown.
The flights are full to the brim these days and TG also is almost the main airline flying asian cargo to kathmandu, so I doubt we'd see the AB300, but i am not sure how a 773 sub would work.