The scenario you've described is an example of Directional Selection. This type of selection occurs when individuals at one extreme of a trait (in this case, height) have a reproductive advantage, leading to a shift in the population's average trait over time. In your example, since the average height of giraffes is increasing due to the availability of food sources higher up, it indicates that taller individuals are favored.
So the correct choice is 4. Directional Selection.