Which organ is known as the body's "blood bank"?

The correct answer is b. Spleen.

The spleen is often referred to as the "blood bank" of the body because it stores a significant amount of blood, which can be released into the circulatory system when needed, such as during a hemorrhage. It also filters blood and removes old or damaged red blood cells.

The final answer is $\boxed{\text{b. Spleen}}$.