Answer:
d) This is a completely randomized design with two explanatory variables (factors).
Step-by-step explanation:
Explanatory variables are independent variables.
In this case, I prepared two explanatory variables, which are : (i)30-second ad
(ii) 60-second ad,
Then, the explanatory variables were assigned to 4 treatment groups, and each variable is done once or thrice.
All subjects are assigned randomly to all treatment groups, with each treatment group seeing one. We can conclude that this is a completely randomized design with two explanatory variables.