Final answer:
The kidneys are the primary organ responsible for regulating the body's fluid and electrolyte balance, maintaining homeostasis by filtering blood and regulating substances like Na+, Ca2+, and K+.
Step-by-step explanation:
The kidneys play a major role in regulating the body's fluid balance. While the skin and lungs contribute to osmoregulation by losing water through sweat and respiration, respectively, it is the kidneys that are primarily responsible for maintaining homeostasis of fluids and electrolytes.
The kidneys filter the blood, reabsorb water and essential nutrients, and excrete wastes in the form of urine. They regulate blood osmolarity by balancing the levels of sodium (Na+), calcium (Ca2+), and potassium (K+). Additionally, they help control blood pressure through the renin-angiotensin-aldosterone system and partner with the lungs and plasma buffers in regulating body pH. Hence, the correct answer is C. Kidneys.