The correct answer to this open question is the following.
According to your question, there are only two parts to be answered, a) and b).
Being that the case, we can comment on the following.
A) This is an observational study and not an experiment because the research was only limited to observation, participants never receive any kind of treatment or another interaction to experiment with them.
B) Who were the subjects of the study?
The subjects of this study were women aged 65 and older who were tracked for several years. The observation revealed that those women who had a vitamin B12 deficiency were twice as likely to suffer severe depression as those who did not.
So, according to the American Journal of Psychology, these were the women observed during the study. There were no more experiments, interactions, or activities applied to them in order to consider it an experiment.
Let's remember that in order to consider an experiment as such, there has to be another component of the study, and the experimental part of it that has to be tested and proved passing the scientific method to conclude in something valid.
