Final answer:
The independent variable in Schachter and Singer's experiment is the injection of epinephrine.
Step-by-step explanation:
The variable described in the experiment is the Independent variable. In Schachter and Singer's experiment, the independent variable is the injection of epinephrine. The researchers manipulated this variable by injecting some participants with epinephrine and others with a placebo. The independent variable is the variable that the researchers intentionally manipulate or change in order to observe its effects on other variables, such as participants' emotional states.