One of the tests used to determine susceptibility to scarlet fever is the:

The Dick Test is a key test historically used to assess susceptibility to scarlet fever, which is caused by the bacterium Streptococcus pyogenes. This test evaluates the individual's immune response to scarlet fever toxin, specifically the erythrogenic toxin produced by the bacteria. If a person is susceptible, a small amount of the toxin is introduced to the skin, and a reaction indicates the person has the potential to develop scarlet fever upon exposure to the actual bacteria.

In contrast, the Schick Test is a method used to determine susceptibility to diphtheria, not scarlet fever. The skin prick test assesses allergies and sensitivities to various substances but is unrelated to infectious diseases like scarlet fever. The ELISA Test is a widely used immunological assay for detecting antibodies, but it's more relevant for diagnosing viral infections or other conditions rather than specifically measuring susceptibility to scarlet fever. Thus, the Dick Test is the most appropriate choice for understanding susceptibility to this particular infectious disease.