Final answer:
Darwinian evolution explains how organisms with advantageous traits are more likely to survive and reproduce.
Step-by-step explanation:
Darwinian evolution, also known as evolution by natural selection, is the correct answer in this case. It explains how organisms with advantageous traits are more likely to survive and reproduce, passing on those traits to future generations. In the example of giraffes with short necks straining to get food, the individuals with longer necks would have an advantage in accessing leaves higher up in the trees, and thus, would be more likely to survive and reproduce.