Final answer:
The hidden variable in the stated observation about elementary students' grades and breakfast consumption is likely to be the socioeconomic status, which impacts access to nutritious food and educational resources.
Step-by-step explanation:
In the statement "Elementary students who eat breakfast have higher grades than those that don’t," a hidden variable that could be influencing the observed difference in grades is c. Socioeconomic status. Socioeconomic status can affect various aspects of a student's life, ranging from their nutritional status to their home environment, access to resources such as technology and books, as well as emotional support from the family. Students from higher socioeconomic backgrounds may have more consistent access to nutritious breakfasts and other educational advantages, which in turn can contribute to their academic performance.