Can someone answer this please?

"Fat cell, yellow bone marrow, femur, nervous system."

Explanation:

Via process of elimination, we can eliminate the first and last answers, as the biggest unit of a single organ system would be the entirety of the system, and those have the system itself jumbled in between the list.

That leaves us with the center-two answers. I believe "bone" is too vague, meaning that bone, in some cases, may be smaller than cartilage, thus, leaving you with the above answer.

the answer to your question is slate