What is the most likely cause of water dripping on a passenger's feet in a vehicle?

Water dripping on a passenger's feet in a vehicle is most likely due to a plugged-up evaporator drain. When the air conditioning system is in operation, moisture from the air condenses on the evaporator coil. This moisture is supposed to drain away through the evaporator drain, typically located underneath the vehicle. If this drain becomes clogged, the water cannot escape and backs up, often leading to water pooling and eventually dripping into the passenger compartment, particularly near the footwell.

A plugged condenser drain would affect the cooling system, but it typically doesn't lead to water in the passenger area. A stopped-up heater core can cause coolant leaks or a failure to heat, but would not typically result in water dripping from the air conditioning system. A stopped-up radiator primarily impacts the engine's cooling function and would not direct water into the passenger area. Thus, the correct answer directly correlates to the function of the evaporator drain and its role in managing condensate from the A/C unit.