Which of the following best illustrates 1 ppm?

The best illustration of 1 ppm (part per million) is indeed represented by one pound of water in one million gallons.

To understand this, it's helpful to think about the concept of ppm as a way of expressing very dilute concentrations of substances. One ppm means that in a million parts (of the solution), there is one part of the substance of interest.

In the context of water, one million gallons is approximately 8.34 million pounds (since one gallon of water weighs about 8.34 pounds). Therefore, one pound of water in one million gallons is effectively saying that there is one pound of a substance in a very large volume of water, fitting the definition of 1 ppm quite well.

The other options do not accurately depict the concept of ppm to the same degree because they involve different units or scales that do not translate directly into the parts per million concept. For instance, one gram of water in one liter is a much higher concentration, as there are 1,000 grams in a kilogram and about 1,000 liters in a metric ton. The equivalences involving gallons and pounds also see variations in the way concentrations are represented, moving away from the simplicity and clarity of one pound in a million gallons.