The question involves understanding the concept of price elasticity of demand along two parallel demand curves AB and CD. To solve this problem, let's consider the underlying principles of elasticity and how they apply to parallel demand curves.

Concept: The price elasticity of demand is a measure of how much the quantity demanded of a good responds to a change in the price of that good. It is mathematically represented as:

\(E_d = \left(\frac{\Delta Q}{Q}\right) \div \left(\frac{\Delta P}{P}\right)\)

Here, the two key points on the diagram are point R on demand curve AB and point S on demand curve CD.

1. The elasticity at a point on a linear demand curve varies along the curve. It is highest (infinite) at the vertical axis, unitary at the midpoint of the curve, and zero at the horizontal axis.
2. Since AB and CD are parallel, they have the same slope. However, point R is to the left of point S, meaning R is at a higher price level and lower quantity demanded compared to S.
3. Price elasticity is greater at higher price levels for linear demand curves. Thus, the elasticity at point R is greater than the elasticity at point S.

The correct answer is that the price elasticity of demand at point R is greater than that at point S. This is because point R lies on a higher price and lower quantity compared to point S on their respective parallel demand curves.