Final answer:
The primary criteria for defining a species in the biological species concept is the ability of organisms to interbreed and produce fertile offspring. The correct option is O c. Ability to interbreed.
Step-by-step explanation:
The primary criteria for determining what defines a species according to the biological species concept (BSC) is c. Ability to interbreed. This concept defines a species as a group of actually or potentially interbreeding organisms. This means that organisms of the same species can mate and produce fertile offspring. While this concept is widely applied, it faces challenges such as determining the interbreeding capabilities of fossilized species or asexually reproducing organisms. The biological species concept stands apart from other species definitions like morphological species concept, which relies on physical similarities, or the phylogenetic species concept, which considers unique characteristics for differentiation.