Which part of black sheep have wool? 

Back 

The correct answer is option 3. Back.

Wool is primarily produced on the body of the sheep, and it is commonly sheared from the sheep's back. The fleece, which is the woolly coat of the sheep, covers the back and sides. The wool is usually harvested by shearing, a process where the fleece is carefully removed from the sheep. The wool from different parts of the sheep's body may vary in texture and quality, but the majority of the wool comes from the back.

While the legs and face of a sheep may have some wool, it is typically not as abundant or desirable for commercial purposes. Therefore, when considering where the majority of the wool comes from on a black sheep, or any sheep for that matter, it is the back that is most significant.