A group of researchers wanted to sort different white blood cell types (monocytes, lymphocytes, and granulocytes) apart from each other based on size differences and to remove unwanted contaminating red blood cells. After a particular manipulation, the red blood cells lysed. The remaining white blood cells increased in size and, more importantly, the size differences among cells increased, allowing for size-based sorting (which requires minimum size differences among cells). What manipulation did the researchers use to increase cell size?

In the given case, the manipulation made by the researchers is that they would have placed the cells in an environment exhibiting lower solute concentration in comparison to the concentration of solute present within the cells.  

When the cells are put in a solution that contains lower solute concentration, that is, in the hypotonic solution then the flow of water will take place from the outside environment to the cell within by the process of osmosis, which will eventually result in the swelling of the cells. Thus, there will be an increase in the size of the cells.  

the sphincter muscles at the end of the digestive tract is the anus.