If the student performs a Benedict's test and observes no color change after adding the reagent and heating it, they can conclude that the unknown substance does not contain reducing sugars. The Benedict's test is a qualitative test used to detect the presence of reducing sugars, such as glucose and fructose.
Normally, if reducing sugars are present, a color change occurs (from blue to green, yellow, orange, or brick red, depending on the concentration of the sugar). Since no reaction occurred in this case, the student can reasonably conclude that the unknown substance either does not contain reducing sugars or that it is not a carbohydrate. Further tests would be necessary to identify the substance more accurately.