Final answer:
Symbiotic relationships are necessary for a variety of species to coexist on the planet, with mutualism being the type where both species benefit.
Step-by-step explanation:
The relationship that must exist between the spheres for a variety of species to exist on the planet is an ecological relationship. However, considering the context of the other options and common biological terminology, the student is likely referring to the relationships between species. In this case, the answer would be symbiotic. A symbiotic relationship is a close, long-term interaction between individuals of different species. There are three types of symbiotic relationships: mutualism, commensalism, and parasitism. In a mutualistic symbiosis, both species benefit from the interaction, in commensalism, one species benefits while the other is neither significantly harmed nor benefited, and in parasitism, one species benefits at the expense of the other.