The answer is number 5: defends the body against pathogens. Red blood cells do not defend the body, white blood cells do. Red blood cells' purpose is to carry oxygen to the rest of our body.
After you realize that you are under stress, the next step is to identify the stressor you are facing. You can do this so you'll be able to recognize how you feel, and maybe even find the cause so you can avoid situations that make you stressed and learn to calm down.