The most logical transition to complete the text is:
(Choice A) Earlier,
So the completed text would read: "Arkansas aviator Louise Thaden was already a record breaker when she won the inaugural National Women’s Air Derby, a race from California to Ohio, in August of 1929. Earlier, in December of 1928, Thaden had set an aviation record when she reached an altitude of 20,269 feet in a Travel Air biplane."
This choice appropriately indicates the chronological order of events.