Which of the Following Would Be Considered a Single-Subject Research

Which of the following would be considered a single-subject research design?

A) the comparison group design whereby outcomes are compared between a group of clients who received the treatment and a group of people who did not get the treatment
B) the AB design where a single client is measured on outcome several times before treatment begins and several times during the treatment period
C) the one-group pretest-posttest design whereby a group of clients are measured on outcome one time before treatment begins and one time at the end of treatment
D) the basic experimental design whereby a group of clients are randomly assigned to either a treatment group or a control group with comparisons on outcome between the two groups