Natalie has Raving Witness and Kadregrin the Infernal in their base. Tara plays Charm on Raving Witness, moving it to the open Forbidding Waste. After Charm resolves with Raving Witness’s movement trigger on the Chain, Natalie plays Thrill of the Hunt targeting Kadregrin the Infernal and plays it to the open Sunken Temple. Which showdown begins first? A. As contested was applied to Forbidding Waste first, that showdown must begin first. B. As contested was applied to Sunken Temple most recently, that showdown must begin first. C. As both contesting units are controlled by Natalie, they choose which showdown to begin first. D. As the turn player, Tara chooses which showdown to begin first.

D — as the turn player, Tara chooses which Showdown begins first.

Both battlefields get their Showdowns Staged in the same Cleanup, once the Chain empties (Raving Witness's move trigger, then Thrill of the Hunt resolving to put Kadregrin at Sunken Temple). Neither battlefield can start its Showdown while a Chain item is still pending — a Showdown only opens from a Neutral Open State (CR 344, 323.11).

When multiple Showdowns are Staged with no Combat staged, the Turn Player picks one of those battlefields and a Showdown begins there (CR 323.11). Whose units applied Contested, and whether both contesting units are Natalie's, is irrelevant to that choice — that only decides Attacker/Defender and Focus inside the Showdown (CR 466.2.c.1). So A, B, and C are all wrong; the order of Contested applications doesn't lock in a start order.