Final answer:
The word 'hindered' fits the sentence since it means to cause difficulty, which would be the effect of a knee injury on someone's ability to race. The other options enhance, strengthen, and maintain do not logically fit the context of an injury.
Step-by-step explanation:
The correct word to complete the sentence is hindered. This word means to create difficulties for someone or something, resulting in a delay or obstruction. Therefore, in the context of the sentence, it indicates that the knee injury caused difficulties for Ken that prevented him from improving his record. The other options, such as enhanced (improved), strengthened (made stronger), and maintained (kept the same), do not make sense in this context because an injury would not improve or strengthen someone's ability to perform physically, nor maintain it if it was a recent occurrence.