writing

Aloe vera is a succulent [plant from whose leaves are extracted] a gel that has been prized since ancient times for its medicinal properties.
(A) plant from whose leaves are extracted
(B) plant from whose leaves is extracted
© plant from whose leaves they extract
(D) plant, from its leaves is extracted
(E) plant with leaves for extracting

Correct Answer: B
My Answer: A

The real subject of that “is” or “are” is “a gel.” Rephrase to see: A gel is extracted from the leaves of the Aloe Vera plant.

I didn’t get why the subject is not leaves

Because the leaves aren’t being extracted. The gel is extracted from the leaves.

You’re pulling the gel out of the leaves. You can’t pull the leaves out of the gel.