If a researcher decides to have a control group in a ‘placebo’ design, he/she should have how many groups?

The correct option is: Two groups.

To understand why a placebo design with a control group requires two groups, we can examine the structure of a standard experimental setup:
1. Experimental (Treatment) Group: This group of participants receives the actual active treatment, medication, or intervention that is being tested.
2. Control (Placebo) Group: This group of participants receives a placebo, which is an inactive substance or sham treatment (such as a sugar pill or saline injection) designed to mimic the active treatment. This group serves as a baseline.

By comparing these two groups, the researcher can isolate the actual biological or therapeutic effect of the treatment from the psychological "placebo effect" (improvements that occur simply because participants expect the treatment to work). Because both groups undergo the exact same procedures except for the active ingredient, any difference in outcomes between the two groups can be attributed to the active treatment. Therefore, the design inherently relies on having two distinct groups.