The correct answer is option 4: Spleen. The spleen is an organ located in the upper left side of the abdomen. One of its main functions is to break down and remove old or damaged red blood cells from the bloodstream.

The process of breaking down red blood cells occurs in the spleen`s red pulp, which contains specialized cells that help with this process. These cells break down the hemoglobin in the red blood cells and recycle components such as iron, which is essential for the production of new red blood cells.

The spleen also acts as a reservoir for iron. When old red blood cells are broken down, the iron from their hemoglobin is stored in the spleen. This stored iron can be released into the bloodstream when there is a need for it, such as during periods of increased red blood cell production or in cases of iron deficiency.

In summary, the spleen is responsible for breaking down old red blood cells and storing iron from them in the human body.