The situation you described is an example of Directional Selection. This occurs when individuals at one extreme of a trait (in this case, height) have a reproductive advantage due to changes in the environment (such as the availability of food in higher branches). Over generations, this leads to an overall increase in the average height of the giraffe population as taller individuals are favored.
Disruptive Selection would involve favoring individuals at both extremes of a trait rather than just one, while Stabilizing Selection would favor intermediate traits. Therefore, the correct answer is:
- Directional Selection.