The correct answer is B. Led
Step-by-step explanation:
The past form of a verb shows an action finished; for example, the past form "wrote" shows the writing process ended. Additionally, the past participle refers to a verb form used as part of perfect tenses. Besides this, in some verbs, the past participle and past form are the same.
In the case of the sentence presented the verb is "lead" and both the past form and past participle of this verb is "led". Besides this, it is appropriate to choose "led" over "lead" because the sentence "The jockey lead the jittery thoroughbred to the starting gate just before the race" suggests the action has finished or it is complete, and therefore a past form should be used.