The most likely impact of reducing low-growing plants from an area is that the proportion of tortoises that could only extend their necks a short distance would increase. This is because, with fewer low-growing plants available, the long-distance neck-extending tortoises would have a feeding advantage, and those that cannot reach the higher food sources may struggle to survive, leading to an increase in the proportion of short-neck tortoises that can access what food is available at lower levels. Therefore, the correct answer is:
The proportion of tortoises that could only extend their necks a short distance would increase.