Let's see--WHY was she so valuable? Starting at the bottom: if she supported suffrage more, that wouldn't help the abolitionists! (D is wrong) Being a New York Quaker doesn't help abolitionists. (C is wrong.) And even if she believed abolition was more important...a lot of people might hold that opinion. That doesn't automatically make her one of the most important people in the movement.
But the reason she's so famous is because she was actually enslaved, and so was able to share her experiences. THIS is what made her special. A is correct!
Answer: She offered the perspective of someone who lived with slavery.