Reducing low-growing plants from an area would likely impact the tortoise population by favoring those with a long-distance neck extension ability, as they would be better adapted to reach food sources that are farther from the ground. As a result, the most appropriate response would be:
"The proportion of tortoises with the ability to extend their necks a long distance would increase."
This outcome is due to natural selection favoring the long-necked tortoises that can feed on higher vegetation, while those with shorter necks may struggle to find sufficient food, potentially leading to a decline in their population.