Answer:
the significance of Douglass' introduction to "The Liberator"? It provided him with knowledge of the anti-slavery movement, as well as a purpose and voice within the movement.
She describes the great risks that she and other slaves took in order to learn.
Out of all the pronouns, "whom" makes the most sense grammatically. This allows us to narrow it down to A or D.
In order for it to be the object of a preposition, there would have to be a preposition in front of it. "For," however is <em>not</em> a preposition. In fact, there isn't a preposition in the whole question.
This leaves us with option D, which is the correct answer.
You can better tell it's a direct object if you flip the sentence around a bit.
Your sister is waiting <em>for whom</em>?
"Whom" is essentially receiving the action. "Whom" is what your sister is waiting for.
Answer: D. whom; direct object.
I was going to answer but big mako already did