Which of the following levels of classification is the most specific?

Species

Domain

Kingdom

Class

it would be species in this case

Your Linnaean system (Classification system) goes like this:

Domain

Kingdom

Phylum

Class

Order

Family

Genus

Species

Genus and species makes up your binomial nomenclature (scientific name). Ex.) Canis Lupus

The Linnaean system goes from the big-to-small.

Thus, species is your smallest level.