Which organism is a teriary consumer in this ecosystem

You did not post the ecosystem involved but below is an explanation of what a tertiary consumer is.

A tertiary consumer is an organism that feeds on other consumers, specifically primary and secondary consumers. They are often carnivores like hawks,owls, and sharks.

In the attachment below, the owl is the tertiary consumer. It mainly feeds of the primary consumers (mouse and rabbit) and the secondary consumer (snake).