Final answer:
OPTION D.
Observing that giraffes with long necks can feed on leaves high on trees and have few competitors offers insight into natural selection; it demonstrates that giraffes with longer necks are likely to survive and reproduce, passing on this trait to their offspring.
Step-by-step explanation:
From the observation that giraffes use their long necks to feed on leaves high up on acacia trees, we can conclude about natural selection that the giraffes with the longest necks survive and produce offspring, as indicated by option (d). We can understand this through the work of Charles Darwin and Alfred Russel Wallace. Their theories explain how over countless generations, the trait for long necks was favored because it allowed those giraffes to access a food source with less competition. Over time, those with longer necks had a survival advantage and were more likely to reproduce, thus passing this advantageous trait to their offspring. This process led to the natural selection of giraffes with long necks.
Alternative hypotheses, such as Lamarck's idea that giraffes' necks became longer over a lifetime of stretching and that this acquired characteristic could be inherited (option a), have been refuted by modern genetics. Observations (option b) about giraffes stretching their necks throughout their lifetime do not lead to inherited changes. The possibility of other animals mimicking the giraffe (option c) is not supported by evidence of natural selection.