Which type of research study compares disease rates among population groups, and attempts to identify related conditions or behaviors?

A. placebo

B. clinical trial

C. case control

D. epidemiological

"Epidemiological" is the one type of research study among the following choices given in the question that compares disease rates among population groups, and attempts to identify related conditions or behaviors. The correct option among all the options that are given in the question is the fourth option or option "D".