Which of the following are part of the body’s first line of defense against pathogens?
a. white blood cells
b. red blood cells
c. lymph nodes
d. mucous membranes

The answer is "D"

The body's first line of defense is also known as Innate immunity and incudes mucous membranes. Both the intact skin and mucous membranes of the body offer a high degree of protection against pathogens.

 A good example is the mucous membrane of the respiratory tract. It is lined up with cilia and covered with mucus. The coordinated beating of the cilia drives the mucus up to the nose and mouth where the trapped bacteria can be expelled.

(Lymph nodes and white blood cells belong to the second line of defense, not the first) 

but there is no

i guess you forgot putting it.

gold i'm pretty sure i might be wrong