The symbol "r" is used to denote distance in the law of universal gravitation, not "d". This is a common convention in physics and related fields, but the choice of symbol is ultimately arbitrary.

Here's why "r" is frequently used:

* Radius: The distance "r" is often thought of as the radius of a sphere surrounding a massive object. The gravitational force acts outward from the center of this sphere, and the radius determines the strength of the force.

* Radial Distance: "r" signifies a radial distance, meaning it's measured directly from the center of one object to the center of another.

* Simplicity: "r" is a simple and easy-to-remember symbol for a fundamental quantity.

While other symbols like "d" could be used, "r" is the standard and widely recognized symbol for distance in gravitational equations. It's crucial to understand the meaning behind the symbol, regardless of its specific character.