Answer: A. breakfast
Step-by-step explanation:
An independent variable is the one which can be altered in an experiment manually. Such changes can be observed on the dependent variable, which cannot be changed by self manipulations instead change with the changes occurring in independent variable. The dependent variable is useful for framing the result of the experiment.
According to the given situation, the breakfast is the independent variable as in the study it is to determine the effect of eating the breakfast have on the concentration on math assignment which is the dependent variable of the experiment.