Answer:
D. A kidney is an organ is correct.
Step-by-step explanation:
A kidney is a part of our body that cleans our blood, ridding it of all uncleanliness, similar to a water filter.
A kidney is an organ because it's one of the essential parts of our body that we need to survive. Our body couldn't live with too much/ too little salt and other substances in our blood.
Therefore, the correct answer is D. A kidney is an organ.
Hope this helps! :D