Final answer:
The hypotonicity of saliva compared to plasma is due to the water impermeability of the ductal cells caused by a relative absence of aquaporin channels.
Step-by-step explanation:
Saliva becomes hypotonic relative to plasma as it flows through the ducts due to the water impermeability of the ductal cells. This impermeability is because ductal cells do not have many aquaporin channels, which are proteins that facilitate water movement across the cell membrane. When stimulated by ADH (antidiuretic hormone), aquaporin channels are inserted into the apical membrane of principal cells that line the collecting ducts, regulating the fluid's osmolarity. In the salivary ducts, a relative absence of aquaporin channels prevents water from being reabsorbed, which makes the saliva less concentrated than the plasma as it travels towards the mouth. Therefore, the correct answer to the student's question is C. water impermeability of the ductal cells.