The organ that primarily maintains water and solute homeostasis in the body is the kidneys.

Here's why:

* Filtration: The kidneys filter blood, removing waste products and excess water and solutes.

* Reabsorption: They reabsorb essential nutrients, water, and electrolytes back into the bloodstream, maintaining a balance.

* Secretion: They secrete additional waste products and excess substances into the urine.

* Regulation of Blood Volume: The kidneys adjust the amount of water reabsorbed to regulate blood volume and blood pressure.

* Regulation of Electrolyte Balance: They control the concentration of electrolytes like sodium, potassium, and calcium in the blood.

While other organs contribute to homeostasis (like the skin, lungs, and intestines), the kidneys are the central organ responsible for regulating water and solute balance.