A keystone species is one that has a disproportionately large effect on its environment relative to its abundance. Among the options you've provided—pine marten, ringtail, frog, and mountain lion—the mountain lion is the most likely to be considered a keystone species.
Mountain lions play a crucial role in their ecosystems as apex predators. They help regulate prey populations, such as deer, which can significantly affect vegetation and, consequently, the entire ecosystem. By controlling the populations of herbivores, mountain lions contribute to maintaining the balance of the ecosystem, influencing species diversity and habitat structure.
While the other species also have ecological roles, the impact of mountain lions on their environments generally qualifies them as a keystone species.