A rainbow option is characterized by a payoff that depends on the best or worst outcome among multiple underlyings (for example, the maximum or minimum of several asset prices).

• A is a classic rainbow-style payoff because it depends on the maximum of multiple assets.
• B is similar because U.S. Treasury bond futures embed a cheapest-to-deliver style optionality across multiple deliverable bonds.
• C is similar because a CDS with multiple deliverable obligations also involves choosing among multiple deliverables.
• D is the exception: a box spread is a synthetic position formed from options and is not a rainbow-style payoff on multiple underlyings.

Therefore, the correct answer is D.