The only type of investigation that involves a control group is a. collective. b. comparative. c. descriptive. d. experimental.

The correct answer is- D) Experimental.

There are primarily three types of investigations, which are descriptive, comparative, and experimental investigations.

Experimental investigation involves manipulation of a particular variable in order to determine any change in another variable.

This method is based on controlled methods ( including control group where no change is done) , so as to test a particular hypothesis and finally draw conclusions.

Thus, option D) is the right answer.