Influenza c is rarely included in vaccine for prevention because:
a. there is no ability to form antibodies.
b. the system are the same as the common cold.
c. it is not spread the same as a or b.
d. it only affects persons who are immunocompromised.

The correct answer is option C.

Explanation:

Influenza C virus is the member of genus Influenzavirus C which causes influenza just like other influenza viruses. These viruses are known to infect pigs and humans. Type C species caused flu is rare in comparison to Types A or B.  

Other common influenza viruses A and B also cause seasonal infestation every year because of their ability to shift antigenicity. Influenza virus C does not have this ability to antigenic shift and not considered as a concerned threat to human health. Because of this, it is rarely included in vaccinations against influenza virus C.

Thus, the correct answer is option C.