In what situation would the female horses be allowed to reach maturity, but the male horses would be killed at a young age?
Take a look at the beginning of the second paragraph. "Herders of domesticated animals used for meat or milk typically kill off all but a few males before they are fully mature, but not the females..."
This matches our question perfectly. This would support the hypothesis that horses were domesticated for meat or for milk. In other words, they were domesticated, but not for riding. This is why answer choice C is correct.