Final answer:
The smallest entity among the given options is the bacterium, which generally ranges in size from 1 to 10 micrometers.
Step-by-step explanation:
The smallest entity among the options provided is the bacterium. Prokaryotic cells, such as bacteria, typically range in size from 1-10 micrometers (µm). In comparison, a human red blood cell is approximately eight micrometers in diameter. Platelets are small fragments of cells, usually smaller than red blood cells, but larger than bacteria. White blood cells are larger than red blood cells and, thus, larger than bacteria and platelets. Onion root cells are eukaryotic cells and are generally larger than human cells. Since bacteria are single-celled organisms and those cells are usually the smallest among the options given, they are considered the smallest.