The coefficient of drag (CD) is a dimensionless quantity that represents the resistance of an object to the motion through a fluid. It is commonly used in fluid dynamics to characterize the aerodynamic properties of objects.

The Reynolds number (R) is another dimensionless quantity that represents the ratio of inertial forces to viscous forces in a fluid flow. It is defined as the product of the fluid density, the flow velocity, and a characteristic length divided by the fluid viscosity.

In this question, we are given that the Reynolds number is greater than 1.9 but less than 500. The question asks us to determine the coefficient of drag (CD) in this range.

To solve this problem, we can use the drag coefficient correlation for laminar flow, which is valid for Reynolds numbers less than 500. For laminar flow, the drag coefficient is given by:

CD = 18.5 / R^0.6

Let's break down the equation and understand its components:

1. CD: The drag coefficient represents the resistance of an object to the motion through a fluid. It is a dimensionless quantity and depends on the shape of the object and the flow conditions.

2. R: The Reynolds number represents the ratio of inertial forces to viscous forces in a fluid flow. It depends on the fluid density, the flow velocity, a characteristic length, and the fluid viscosity.

3. 18.5: This constant is specific to the equation and represents the correlation between the drag coefficient and the Reynolds number for laminar flow.

4. R^0.6: This term represents the Reynolds number raised to the power of 0.6. It accounts for the influence of the Reynolds number on the drag coefficient.

Based on the given equation, we can see that the drag coefficient (CD) is inversely proportional to the Reynolds number (R). As the Reynolds number increases, the drag coefficient decreases.

Therefore, when the Reynolds number is greater than 1.9 but less than 500, the correct equation to calculate the coefficient of drag (CD) is:

CD = 18.5 / R^0.6

Hence, the correct answer is option 'C'.