Option C: We slip on muddy roads due to lack of friction. Friction is the force that opposes motion between surfaces in contact. Muddy surfaces reduce friction because the water and soil particles act as lubricants, decreasing the grip between our shoes and the ground. Consequently, when we apply force to move forward, the insufficient friction fails to provide the necessary traction, causing us to slip. Options A (Gravitational force) and B (Relative velocity) are not directly related to slipping on muddy roads. While gravity plays a role in keeping us grounded, it doesn’t cause slipping. Relative velocity refers to the velocity of one object relative to another and is not relevant in this context. Option D (Excess of friction) is also incorrect; excess friction would provide more grip, reducing slipping. Therefore, the primary reason for slipping on muddy roads is the lack of friction, which is crucial for maintaining traction and preventing slipping on various surfaces.