A 34-year-old woman with posttraumatic stress disorder has been treated with exposure therapy, with partial success. Her clinician is considering an adjunct medication. The agent d-cycloserine could be efficacious for reducing symptoms in anxiety disorders because it has been shown to:

a) Inhibit GABA receptors
b) Enhance extinction learning
c) Increase serotonin reuptake
d) Block NMDA receptors