In dry regions, trees undergo certain adaptations to cope with the limited water availability. One of these adaptations is reducing the size of their leaves. The process behind this is to reduce transpiration.

Option 1: reduce metabolism - This option is not correct. While reducing metabolism may help conserve energy, it is not directly related to the reduction in leaf size in dry regions.

Option 2: reduce transpiration - This option is the correct answer. By having smaller leaves, trees can reduce the surface area through which water is lost via transpiration. This adaptation helps them conserve water in a dry environment.

Option 3: maintain natural growth - This option is not correct. The reduction in leaf size is not for the purpose of maintaining natural growth but rather to adapt to the limited water availability.

Option 4: protect plant from animals - This option is not correct. While smaller leaf size may provide some protection against herbivores, it is not the primary reason for the reduction in leaf size in dry regions.

Overall, the correct answer is option 2 because reducing leaf size helps to reduce transpiration and conserve water in dry regions.