A red blood cell will shrink in size when placed in a more concentrated salt solution because of the passive process called

Osmosis

Explanation:

In a highly concentrated salt solution, concentration of water is comparatively lower than the fluid inside red blood cells. This develops a concentration gradient.

Due to this difference in concentration, the water starts flowing out of the red blood cells across its permeable layer. Due to this movement of water, red blood cell shrinks. This physical phenomenon in which a substance move from a region of high concentration to a region with lower concentration across any permeable membrane is called Osmosis.

This process is called osmosis whereby water exits the cell through its selectively permeable membrane and into the salt solution to dilute and balance the pressure caused by the concentrated salt solution.