RBC will __________ if kept in 0.8% NaCl

The correct option/answer is "remain unchanged".

To understand why a Red Blood Cell (RBC) remains unchanged in a 0.8% NaCl (sodium chloride) solution, we must look at the concepts of osmosis and tonicity.

Osmosis is the net movement of water molecules across a semipermeable membrane from a region of lower solute concentration (hypotonic solution) to a region of higher solute concentration (hypertonic solution) until equilibrium is reached.

The internal cytoplasm of a human red blood cell has an osmotic pressure equivalent to an aqueous solution of approximately 0.9% NaCl (often reported in a range around 0.85% to 0.9%). A solution containing 0.8% to 0.9% NaCl is considered isotonic to the cytoplasm of red blood cells.

In an isotonic solution, the concentration of solutes outside the cell is equal to the concentration of solutes inside the cell. Therefore, the rate of water molecules entering the cell is equal to the rate of water molecules leaving the cell.

Since there is no net movement of water into or out of the RBC, the cell maintains its normal shape and volume, meaning it will remain unchanged.