I agree that the "technical rules" say that player C's action was a call of 1200. However, it is a extremely rare for a player with less than 2 BBs left in his stack to push his last two chips into the pot silently, while UTG, expecting to receive change for less that than a single BB, while knowing that he will be forced to be truly all-in the very next hand, should he lose this hand. Sure, it could happen if the player was a novice - but would the novice know that his silent action should be interpreted as a call given the "technical rules?" It's getting harder to believe.
Now, player D calls 2000. The original post (OP) doesn't tell us "how" player D made the call. Was it verbal? Did it involve 4x500 in chips? In any case, if it was clearly meant as a call of 2000, then player C MUST speak up at that point to clarify his action as being only a call of 1200. Otherwise, I'm using Rule #1 to override the "technical rules" as noted above.
Regards,
B~