Which describes how blood can be protective in nature?
a. by forming a protective layer within blood vessels
b. by carrying cells that defend the body from foreign materials
c. by secreting oil that destroys foreign materials
d. by producing acid that kills foreign materials

The answer is B. by carrying cells that defend the body from foreign materials.

The blood is not able neither to form a protective layer within blood vessels, nor to secrete oil that destroys foreign materials, nor to produce acid that kills foreign materials. 
But, blood does carry some types of leukocytes (white blood cells) that defend the body from foreign materials. They fight germs (viruses and bacteria) by producing antibodies. Antibodies are special proteins that recognize special molecules on the surface of the foreign cells. Attaching the antibodies to the antigens helps the body destroy or neutralize foreign materials. That's why when a person has an infection, his leukocyte count in the blood is much higher.