What does the term natural selection mean

the process whereby organisms better adapted to their environment tend to survive and produce more offspring. The theory of its action was first fully expounded by Charles Darwin and is now believed to be the main process that brings about evolution.

Natural selection is the basically the process of "survival of the fittest" in which better adapted organisms will succeed overall. 

The correct answer is - traits for smaller animals with short horns will be selected.

Because of the pressure from the hunters that will be put upon the animals that are larger and have big horns, the animals will respond in having smaller size and smaller horns, or even be without horns. This will be an evolutionary trait in order for the animals to be able to survive, as they will be witnessing the death of the larger ones with bigger horns, so the selection will be going in the opposite way as the chances for survival are greater.

The example with the elephants in South Africa is a perfect example, where the elephants started to have much shorter tusks, or no tusks at all, after they were hunting because of the tusks.