Which of the Following Is the Best Example of a Correlational

Which of the following is the best example of a correlational research study?

A) Researchers assigned participants to do either a stressful task or an easy task and then measured their blood pressure.
B) Researchers compared the blood pressure of participants that were trained to reduce stress by meditating and participants that were trained to reduce stress by drawing.
C) Researchers measured the stress levels in participants that were given a blood pressure drug and participants that were given a placebo drug.
D) Researchers measured the association between participants' blood pressures and their degrees of chronic stress.