**Core Concept**
The glomerular filtration rate (GFR) is the rate at which the kidneys filter waste and excess fluids from the blood. Renal plasma flow (RPF) is the volume of plasma that flows through the kidneys per unit time. The percentage of GFR to RPF is an important concept in understanding renal physiology.

**Why the Correct Answer is Right**
The glomerular filtration rate is approximately 20% of the renal plasma flow. This is because the glomeruli, which are the filtering units of the kidneys, have a high permeability to water and small molecules, allowing most of the filtered fluid to pass through into the nephrons. The remaining 80% of the renal plasma flow is either reabsorbed back into the bloodstream or excreted in the urine. The glomeruli are responsible for the filtration of the blood, and the tubules play a crucial role in the reabsorption and secretion of ions and water.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too high, and the actual percentage of GFR to RPF is less than 100%.
**Option B:** This option is incorrect because it is too low, and the actual percentage of GFR to RPF is more than 10%.
**Option C:** This option is incorrect because it is a percentage, but it is not the correct percentage of GFR to RPF.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the glomerular filtration rate is a critical indicator of kidney function, and any changes in GFR can indicate underlying kidney disease.

**Correct Answer: D. 20%**