Final answer:
The sentence "The study used a 2 x 2 x 4 design" indicates the design of the study with multiple factors and their levels.
Step-by-step explanation:
The sentence "The study used a 2 x 2 x 4 design" indicates the design of the study, which involves multiple factors. The numbers in the design represent the levels of each factor. In this case, there are three factors: two with two levels and one with four levels. Each factor represents an independent variable, and the levels represent the different conditions or treatments being tested. Therefore, the correct answer is c. The number of participant variables.