Answer:
Placebo effect
Step-by-step explanation:
Placebo effect occurs when an individual starts to show positive response to an inactive substance after being told the substance has powers to cure.
The person's mind subconsciously helps him heal or perform better on the false belief that the substance is effective.
In the given scenario Jeanne labelled decaffeinated coffee as caffeinated coffee. On consumption her co-workers claimed that the extra boost of caffeine helped them focus on their work.
This is a placebo effect.