The correct answer is: Paul's immune system functioning will improve.
Prolonged stress suppresses and negatively affects the functioning of a person or animal's immune system. Due to this, a person is more susceptible to infectious diseases such as colds, gastroenteritis, etc. Stress suppresses immune system functioning due to increased levels of cortisol, which in turn lowers the number of lymphocytes (antibodies that fight illness and infection. If Paul's lifestyle changes from a highly stressful one (as a stockbroker), to a more relaxing one (running a yoga studio in the tropics), his immune system functioning will improve.