Reducing low-growing plants from an area would likely impact the tortoise population by favoring the variety of tortoises with the ability to extend their necks a long distance. The reasoning behind this is that with fewer low-growing plants available as a food source, tortoises that can reach higher vegetation would have a competitive advantage.
Therefore, the best response is:
The proportion of tortoises with the ability to extend their necks a long distance would increase.
This response indicates that long-necked tortoises would be better suited to survive and reproduce in an environment where low-growing plants are sparse.