What does the figurative language convey in this line from the autobiography?
and, cruel as the deed may strike any one to be, for a man to sell his own children to human flesh-mongers, it is often the dictate of humanity for him to do so;
(from Narrative of the Life of Frederick Douglass An American Slave Written by Himself by Frederick Douglass)

a. power of society
b. support of society
c. weakness of the individual
d. inhumanity of the individual