Do teenagers prefer yellow candy or pink candy? Two different
supplements, which have no effect on taste, are used on identical candies
to produce a yellow or pink candy. Volunteer teenagers are randomly
assigned to either pink candy or yellow candy. The participants are then
asked to rate the candy on a 1- to 10-point scale. Is blinding demonstrated
in this experiment? (4 points)

No, because the volunteers know whether they are eating yellow or pink candy.

Step-by-step explanation:

First of all I just took the exam and got it right. Secondly, in a single blind trial, the participants do not know what group they have been assigned to nor what treatment (or candy in this case) they are receiving. The question never stated that the teenagers wore a blindfold or were unaware of the group they were in so we assume they were aware of the type of candy they were eating.