What type of force causes repulsion between the surfaces of two negatively charged cells?

The correct answer is based on the fundamental principles of electrostatics. When two cells carry a negative charge, they experience repulsive interactions due to the electrostatic force. This force arises from the Coulomb's law, which states that like charges repel each other, while opposite charges attract. Since both cells in this scenario are negatively charged, the electrostatic force causes them to repel one another.

Other forces mentioned in the question do not account for the specific repulsion observed between negatively charged surfaces. Steric forces relate to the physical presence of molecules impacting each other's spatial arrangement but do not primarily explain the repulsion of charged particles. Binding forces typically involve interactions between molecules or cells that lead to adhesion rather than repulsion. Mechanical forces pertain to physical interactions based on stress or strain and do not specifically address the electrostatic interactions that are crucial in the context of charged cells. Thus, electrostatic force is the most accurate description of the repulsion between two negatively charged cells.