Delta's options to counter a hypothetical United-JetBlue merger are even more limited:

• No Major U.S. Airline Remains Unaligned: All other large U.S. carriers (American, Southwest, Alaska, and now the combined Alaska-Hawaiian) are either too large for regulatory approval or already merged.
  
• Regional/Niche Carriers: Delta could look at smaller regional airlines, but these would not provide the scale or network boost of a JetBlue-United combination.
  
• International Partnerships: Delta may deepen or expand its global alliances (such as with Air France-KLM, Virgin Atlantic, LATAM, or WestJet) to strengthen its international reach.
  
• Organic Growth and Product Investment: With few domestic merger options, Delta could focus on expanding its own network, improving its product, and investing in loyalty or technology to differentiate itself.
  

In summary, with Alaska having acquired Hawaiian, Delta has no obvious large-scale U.S. merger partner left. Its most viable responses would be expanding international partnerships or investing in organic growth, as a major domestic merger is highly unlikely under current regulatory conditions.