Answer:
The answer is directional selection.
Step-by-step explanation:
Directional selection is a type of natural selection, which is understood as favoring an organism through evolution in order to adapt to its surroundings.
Directional selection occurs when an extreme phenotype, which is the characteristic the individual shows, is favored and thus can be expressed in the general population with more frequency, since it is useful.
In this case, long necks are useful for the giraffes, since they help them reach the high leaves and also favor their fighting skills. Overtime, the long neck became the dominant phenotype amongst these animals.