A study was conducted to evaluate the impact of taking a nutritional supplement on a person's reaction time. One hundred volunteers were placed into one of three groups according to their athletic ability: low, moderate, or high. Participants in each group were randomly assigned to take either the nutritional supplement or a placebo for six weeks. At the end of the six weeks, participants were given a coordination task. The reaction time in completing the task was recorded for each participant. The study compared the reaction times between those taking the supplement and those taking the placebo within each athletic ability level. Which of the following is the best description of the study?
(a) A randomized block design
(b) A completely randomized design
(c) A matched-pairs design
(d) A randomized observational study
(e) A stratified observational study

A randomized block design has the following procedure: The researcher first divides the sample into different subgroups called "blocks" where each block has similar characteristics (The block variability is less than the variability between blocks), then, the researcher applies different treatments to people in each block, the subjects within each block are selected at random to receive these different conditions.

In the example, the researchers first divided the volunteers into three groups according to their athletic ability (these would be the blocks) and then, they randomly were assigned a different treatment (supplement or placebo). Finally, they were assigned a task and observed the results. Therefore, it is an example of randomized block design. Thanks to how the study was made, it ensures less confounding variables.