An organism occupying the 3rd trophic level is best described as a tertiary consumer secondary consumer top carnivore predator keystone species

The answer is: Carnivores
Organisms at the third trophic level are secondary consumers. These secondary consumers prey on the primary consumers which are animals that eat plants or herbivores. Secondary consumers are best described as carnivores because they don't eat plants. They only eat animals that consume plants like cows and chicken.