The correct answer is A.
The dependent variable is: how long students worked on the math problem. Secondly, the goal of the study is to assess whether an explanatory variable (the confort of chairs) does have a causal effect on the lenght of math study or not.
As it is described in the question, the explanatory variable is a binary one: it takes value equal to 1 for the group seated in padded, comfortable chairs, and value 0 for the individuals in the group with standard chairs. Therefore, the study can be understood as the assesment on the validity of a treatment, on its capacity to make a difference on the dependent variable.