Which sentence correctly uses a pronoun in the possessive case as an adjective? i feel much safer performing stunts on hers black horse. i feel much safer performing stunts on mine black horse. i feel much safer performing stunts on our black horse. i feel much safer performing stunts on yours black horse.

Correct  "I feel much safer performing stunts on our black horse."

Why the other answers do not work: 

"I feel much safer performing stunts on hers black horse" should be corrected to "her black horse." Adding an "s" to the word her does not make it possessive as the word "her" is already possessive.

In the case of "...mine black horse, " this sentence uses mine which is an objective possessive pronoun. "This is mine." You should use "my" instead: "...on my black horse."

In the final case, "yours black horse" is incorrect because the word "your" is already possessive. Correction: "your black horse."