Which of the following is considered a solid organ?

The kidneys, heart, and spleen are all classified as solid organs, which are characterized by having a defined structure and being capable of performing specific physiological functions. Solid organs are typically involved in processes such as filtration, secretion, and hormonal regulation.

The heart is a vital organ responsible for pumping blood throughout the body, facilitating circulation, and ensuring that oxygenated blood reaches tissues and organs. It is a muscular organ that works continuously to maintain blood flow.

The kidneys play a crucial role in the body's filtration system, removing waste products from the blood to form urine and regulating fluid and electrolyte balance. They also secrete hormones that are important for blood pressure regulation and red blood cell production.

The spleen is involved in the immune system and blood filtration. It helps to filter out old or damaged red blood cells and participates in the immune response by producing lymphocytes and filtering pathogens from the blood.

Given that all of these organs are classified as solid organs and share the common characteristic of being integral to various body functions, the answer that covers all of them together is indeed the most comprehensive choice.