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The kidneys play a crucial role in maintaining homeostasis in the body, which encompasses a variety of functions that directly affect the overall health and well-being. When considering the statements, it becomes clear why all are true.

The kidneys indeed regulate the composition of the blood by filtering out waste products and excess substances, which helps ensure that the electrolyte levels, pH, and osmolarity of the blood remain within a narrow range. This is vital for normal cellular function, as any significant deviation can disrupt physiological processes.

In addition to composition, the kidneys are also responsible for regulating the volume of the blood. They manage this through the processes of filtration and reabsorption, adjusting water output based on the body’s needs. For instance, if hydration levels are low, the kidneys will conserve water, leading to a more concentrated urine output and thus a higher blood volume.

Furthermore, the kidneys participate in the activation of vitamin D, which is essential for calcium absorption and bone health. They convert inactive vitamin D into its active form, calcitriol, through a series of enzymatic reactions triggered in response to parathyroid hormone levels and serum calcium levels.

Given that each of these functions is indeed performed by the kidneys, the comprehensive choice encompassing all these statements reflects