When Tracy was a young child, she often complained of stomachaches at bedtime. Her parents discovered that if they gave her the “green medicine” she always felt better immediately. The green medicine consisted of water with green food coloring. This is an example of
stress sensitization
the placebo effect
a paradigm shift
selective attention

The placebo effect.

Explanation:

A placebo effect can be defined as a phenomenon that acts like a treatment and offers benefit in health but in reality is fake. A placebo effect can be defined as anything that looks like to be a medical treatment, but in reality it is not. Though the treatment given is not real, yet it have positive affect on the body. In this treatment, mind is tricked by fake-treatment.

In the given case, Tracy is being given the 'green medicine' that was nothing but water with green food coloring. Though this treatment is not real but gave benefit in Tracy's health.

So, the correct  answer is the placebo effect.

we need to see the examples!