A human amazed the world in 2004 [by winning] the Man versus Horse Marathon, [it] has pitted hundreds of people [against] horses every year [year since] 1980. [ No error]

i picked (B) becausee of the run on but i don’t know what should be replaced with it !

Replace the “it” with “which.” That relative pronoun makes it so that the second clause is dependent on the first to tell you what the “which” is.