Final answer:
The brain, specifically the hypothalamus, is the primary organ responsible for regulating homeostasis, while the kidneys also contribute significantly to maintaining internal balance.
Step-by-step explanation:
The organ that is primarily responsible for monitoring and regulating internal conditions to maintain homeostasis is the brain. Specifically, the hypothalamus within the brain serves as the body's thermostat, playing a crucial role in maintaining a stable internal environment by regulating temperature, thirst, hunger, and other homeostatic processes. The kidneys also play significant roles in homeostasis, such as filtering blood, removing wastes, regulating blood pressure, osmolarity, and electrolyte balance, especially Na+, Catt, and K+. However, when it comes to the primary regulation of homeostasis, the brain, particularly the hypothalamus, holds the key role.