An antigen
a. could be an invading bacterium is a protein molecule that defend the body against disease
b. is a foreign molecule that evokes a specific response by a lymphocyte
c. is a protein attacked by an invading microorganism
d. induces development of white blood cells in the bone marrow

b. is a foreign molecule that evokes a specific response by a lymphocyte

Explanation:

An antigen is a molecule (usually proteins) capable of stimulating an immune response, they can be substances from the environment (chemicals, virus, bacteria), or form inside the body. Each antigen had distinct surface features (epitopes) that result in specific responses. Depending on the antigen presented, several types of immune cells can become activated, lymphocytes included.

Considering this information we can conclude that the correct answer is b. is a foreign molecule that evokes a specific response by a lymphocyte.

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