The correct answer is: C. 18 to 26.
Sodium absorption ratio (SAR) is a measure of the relative sodium content of water. It is calculated by dividing the concentration of sodium in the water by the concentration of calcium and magnesium in the water. A SAR of 18 to 26 indicates that the water has a high sodium content.
Water with a high sodium content can be harmful to people with certain health conditions, such as high blood pressure and kidney disease. It is important to check the SAR of your drinking water if you have any of these conditions.
Option A is incorrect because it indicates a low sodium content. Option B is incorrect because it indicates a moderate sodium content. Option D is incorrect because it indicates a very high sodium content.