D. This is an experiment because it is applying a treatment
<h3>Step-by-step explanation:</h3>
Experiments and observational studies are both ways to find new information about a hypothesis but in different ways.
Vocabulary
First, to understand the question, let's define the key terms.
Experiment - An experiment is when a scientist purposely applies a treatment and interferes with a person's life in order to gather information.
Observational Study - This type of study attempts to study a person without interfering with their lives in any way.
These definitions eliminate the answer choices A and C because both of them match the incorrect definition of the term.
Experiment vs. Observational Study
Now, we need to figure out if this specific example is an experiment or an observational study. In this case, the nutritionist is giving a treatment of vitamins. This means that this cannot be an observational study because the scientist is interfering with people. It has to be an experiment because there is a specific treatment being applied.
