What distinguishes sexual reproduction from asexual reproduction?

The distinction between sexual reproduction and asexual reproduction primarily lies in the involvement of gametes. Sexual reproduction necessitates the fusion of specialized reproductive cells known as gametes, which typically come from two parents. This process results in offspring that have genetic contributions from both parents, leading to increased genetic diversity within a population.

In contrast, asexual reproduction does not involve the fusion of gametes; rather, it often involves a single organism replicating itself. As a result, the offspring produced are genetically identical to the parent, which typically limits genetic variation. This core characteristic makes sexual reproduction a key mechanism for introducing genetic variation in a species, which can be crucial for adaptation and evolution.

The other options in this question do not accurately define the principal difference between sexual and asexual reproduction. For instance, asexual reproduction does not inherently result in genetic diversity, and it can occur in a variety of organisms, including single-celled ones. Furthermore, sexual reproduction is not exclusive to multicellular organisms, as many single-celled organisms also reproduce sexually. Thus, the requirement for gamete fusion is what clearly distinguishes sexual reproduction from asexual reproduction.