In the article,"Cognitive costs of exposure to racial prejudice" by Salvatore and Shelton, what is the dependent variable and how is it...

The dependent variable that was studied by Salvatore and
Shelton was cognitive ability, or the capacity to exert mental effort.  This dependent
variable was measured by means of the "Stroop Test."

In a
Stroop Test, subjects are shown the names of colors (like red or blue) in colored
letters.  The color of the letters does not match the name of the color given.  So, for
example, the word "blue" might appear in red letters.  It takes mental effort to see
past the word (blue) and concentrate on the color of the letters
themselves.

What Salvatore and Shelton did was to see how
being exposed to racist decisions affected people's performance on the Stroop Test. 
Since the Stroop Test measures cognitive ability, the dependent variable was cognitive
ability and it was measured via the Stroop Test.