Reducing low-growing plants from an area would likely impact the tortoise population in several ways, but primarily it would affect the availability of food sources. Tortoises that can extend their necks long distances would have a competitive advantage when foraging for food, as they would be able to reach higher vegetation that is still available.
Given this scenario, the most likely outcome is that the proportion of tortoises with the ability to extend their necks a long distance would increase. This is because individuals with longer necks would be better suited to the altered environment, allowing them to find more food and survive more effectively than those with shorter necks.
Conversely, the variety of tortoises with the ability to extend their necks a long distance would likely not go extinct, as they would have a greater chance of survival.
The other options—an increase in short-necked tortoises, or their extinction—would be less likely given that they would have limited access to food in a landscape with reduced low-growing vegetation. Therefore, it is reasonable to conclude that the selective pressure from the loss of low-growing plants would favor the long-necked tortoises.