A keystone species is one that plays a critical role in maintaining the structure of an ecosystem. Among the options provided, it is typically the predator or a species that has a substantial impact on the ecosystem's biodiversity.
In this case, without additional context from the food web, the frog is likely to be the most suitable answer. Frogs often serve as both predators and prey in their ecosystems, influencing populations of insects and providing food for larger animals.
If you have additional context or specific interactions from the food web, that could further clarify which species specifically is a keystone. However, based on general ecological roles, the frog is a reasonable choice.