Because gretchen is afraid of contracting infectious diseases, she compulsively avoids shaking people's hands or touching doorknobs. research suggests that an especially effective treatment for her difficulty would involvea. client-centered therapy. b. psychodynamic therapy. c. cognitive therapy. d. behavior therapy.

Behavior therapy.

Explanation:

Behavior therapy may be defined as the therapy refers to the clinical psycotherapy that uses the technique derived from the behaviorism.  This theory was developed by  Edward Thorndike.

Cognitive behavioral therapy includes behavioral psychotherapy and  cognitive psychotherapy. This therapy can be used to treat the individual's behavior against a particular thinking. The behavior and attitude of the individual can be improved by behavior therapy.

Thus, the correct answer is option (9).