The option that would most negatively affect the export economy of the United Kingdom is:
C. A deadly disease spreads among livestock.
This scenario could lead to significant disruptions in the agricultural sector, particularly affecting livestock exports and related industries. An outbreak could result in trade bans, a decrease in livestock production, and increased costs for disease management, all of which would negatively impact the export economy.
While the other options could have various economic impacts, option C is the most directly harmful to a key sector in exports.