Final answer:
The experimental unit is the individual piece of fabric treated with a particular combination of stain type, removal ingredient, and water temperature.
Step-by-step explanation:
The experimental unit in the described research is the individual piece of fabric that is subjected to one combination of treatment: a particular stain type, one of the stain removal ingredients (baking soda or bleach), and a specific water temperature (cold or warm). Each of these pieces will receive a particular treatment and will be scored for its stain removal quality. This setup allows the experimenters to assess the effectiveness of different combinations of ingredients and water temperatures on stain removal.