Final answer:
Professor Diaz's study on the impact of sugar consumption before bedtime on children's activity levels features sugar as the independent variable, while the children's activity level would be the dependent variable.
Step-by-step explanation:
Professor Diaz is conducting a study to determine whether eating sweets for a bedtime snack makes children more active. In this experimental context, the amount of sugar given before bedtime would be the independent variable. This is the factor that is being deliberately changed by the experimenter to observe the effects it produces on the subject's activity level, which would be the dependent variable.
The control condition would be the circumstances or setup in which no sweets are given, allowing for a comparison between the experimental and control groups. A placebo is an inert treatment that is used to control for the effects of participant expectation, which is not applicable in this particular context, as the action of eating sweets and their content is directly being studied.