Answer;
They contain structures that perform life-functions
Single celled organisms are able to maintain internal stability because they contain structures that perform life-functions.
Explanation;
Single cell organisms are those organisms that contain only one cell unlike multicellular organisms that contain many number of cells.
Single cell organisms include bacteria, archaea and protozoa.
Single cell organisms live and carry out all their life processes in one single cell. Most of them are microscopic; examples are Euglena, green algae, etc.