Why I which activity would stimulate the human immune system to provide protection against an invasion by a microbe

The question is incomplete as it does not have the options which are:

(1) receiving antibiotic injections after surgery

(2) choosing a well-balanced diet and following it throughout life

(3) being vaccinated against chicken pox

(4) receiving hormones contained in mothers milk while nursing

Being vaccinated against chicken pox

Explanation:

Vaccination is the procedure of giving the disease-causing germs which could be virus and bacteria in their dead or weakened state, some toxin, some proteins or some other molecules called a vaccine.

The benefit of providing vaccination is that the provided pathogen acts as an antigen and triggers the proactive immune response against the pathogen which helps in eradicating the main pathogen that has invaded the body.  The vaccination is used to treat the disease like chickenpox, measles, tetanus and many others.

Thus, the selected option is correct.

they are made of gases.

they have a spherical shape.

they make energy through fusion.