Definition of sourcing?

i'm a frog i'm a frog i read about frogs i breath thee frogs the frogs are great but not me

the correct answers are:

c. “never in the history of his people had a man married a woman who spoke a different tongue.”

d. “‘it has never been heard,’ was the verdict of an old man speaking a few weeks later.”

e. “this man had come with others to commiserate with okeke.”