The correct answer is d) proteins.
The kidneys play a vital role in filtering blood and forming urine. The filtration membrane of the kidneys is designed to allow certain substances to pass through while blocking others based on their size and charge.
Water, urea, ions, and glucose are small enough to pass through the filtration membrane. Proteins, on the other hand, are generally too large to pass through this barrier. This is crucial because the presence of proteins in urine is an indication of kidney damage or disease, as healthy kidneys should retain these large molecules in the bloodstream.
In summary, while water, urea, ions, and glucose can easily pass through the filtration membrane, proteins are typically prevented from doing so due to their size.