Final answer:
Species within the same family (Option C) are the most closely related among the given taxonomic levels. They share a more recent common ancestor compared to species in the same order, class, phylum, kingdom, or domain.
Step-by-step explanation:
The taxonomic levels, from the most general to the most specific, are: domain, kingdom, phylum, class, order, family, genus, species. When considering which species are the most closely related, look for the smallest, most specific taxonomic level in question. Among the options provided, species within the same family are the most closely related. Therefore, the correct answer is C. Family.
The reason family is the correct answer is that as you go down the taxonomic hierarchy from domain to species, the organisms classified within each level are more closely related to each other. Members of the same family are more closely related than those in the same order, class, phylum, kingdom, or domain. Within a family, there can be one or more genera, and within each genus, there can be one or more species. All of these levels reflect increasing specificity and closer evolutionary relationships.