A keystone species is one that has a disproportionately large impact on its environment relative to its abundance. Among the options you provided, the frog is most likely to be a keystone species. Frogs often play critical roles in their ecosystems as both predators and prey, affecting the populations of insects and smaller creatures, and are also indicators of environmental health.
If you have a specific food web in mind, please provide more details to refine the answer further!