The major difference between a case study and a single-case experimental design study is that:

A) case study participants are subject to the manipulation of variables.
B) single-case experimental design studies involve naturalistic observation.
C) case study participants' behaviors are measured by some standard method.
D) single-case experimental design study participants receive some type of intervention or manipulation, and behaviors are measured more than once.

The major difference between a case study and a single-case experimental design study is that single-case experimental design studies involve manipulating an independent variable or providing an intervention and measuring the effects on behavior, whereas case studies simply describe a person, group, or situation without manipulation or intervention. In single-case experimental design studies, behaviors are measured repeatedly over time to establish a functional relationship between the intervention and the behavior. Therefore, choice D is the correct answer as it captures this key difference between the two types of studies.