Which of the following examples best illustrates asexual reproduction?

Asexual reproduction is a method by which a new organism is created without the involvement of sexual processes, meaning there is no fusion of gametes. The example of cutting a stem and planting it clearly demonstrates this process, as it involves taking a part of the plant (the stem) and replanting it to grow a new, genetically identical individual. This method is commonly used in horticulture and gardening, known as vegetative propagation, where various parts of the plant can develop roots and grow into a new plant.

In contrast, the other choices involve sexual reproduction or processes that do not define asexual reproduction. Cross-pollination of flowers relies on the transfer of pollen from one flower to another, leading to the fusion of male and female gametes, which is a sexual reproduction mechanism. Spore formation in ferns can be somewhat related to asexual reproduction, as ferns can reproduce using spores, but the actual life cycle involves both sexual and asexual stages. Seed dispersion by wind is a method of distributing seeds formed through sexual reproduction, as it relies on fertilized ovules developing into seeds. Therefore, the act of cutting a stem and planting it is the best example of asexual reproduction in this case.