Final answer:
Eating too much salt leads to water retention through osmosis, the movement of water across a membrane to balance solute concentration, facilitated by aquaporins. Option 3 is the correct answer.
Step-by-step explanation:
Eating too much salt and retaining water is a good example of osmosis, which is the transport mechanism where water moves across a semipermeable membrane from an area of low solute concentration to an area of high solute concentration.
The aim is to equalize the concentrations on both sides of the membrane. This process is facilitated by special proteins called aquaporins that allow the passive diffusion of water. The opposite process, wherein particles move across a membrane from an area of high to low concentration without energy expenditure, is known as facilitated diffusion.
However, when cells actively move substances against their concentration gradient, this involves active transport, which requires energy. Lastly, filtration is influenced by pressure and involves substances moving through a membrane according to their concentration gradient, often seen in the kidneys where blood pressure forces substances into renal tubules.