The correct response would be:
The proportion of tortoises that could only extend their necks a short distance would increase.
Reducing low-growing plants in an area means food resources that are accessible to short-necked tortoises would be limited. Long-necked tortoises would have an advantage as they can reach food that is higher up, so the short-necked variety may struggle more for resources, ultimately reducing their population. However, if the low-growing plants are greatly reduced and no other food sources are available, the long-necked tortoises could benefit, potentially allowing some of the short-necked tortoises to survive if they can adapt to different food sources. Nonetheless, the initial impact would favor the long-distance neck extension ability.