What differentiates a function's prototype from an object's prototype?

A function's prototype is indeed defined by the function itself, which is a fundamental aspect of JavaScript's object-oriented design. When you create a new function in JavaScript, it has a default property called prototype that is used for inheritance. This prototype is what allows instances of that function (when treated as a constructor) to inherit properties and methods through the prototype chain.

This behavior differentiates the concept of a function's prototype from that of an object's prototype. While a function's prototype is inherently tied to the function itself, an object's prototype is derived from its constructor function or can be altered through prototype chaining. Understanding this distinction is crucial when working with JavaScript's inheritance model and the prototype chain, which facilitates method sharing and property inheritance among objects created via constructors.

The other options address different aspects of how prototypes work in JavaScript. For example, while a function's prototype can indeed be altered at runtime, this alone does not encapsulate the unique relationship that a function has with its prototype. Meanwhile, the nature of an object's prototype being an instance of a function or holding its properties reflects more about the mechanics of JavaScript objects rather than the fundamental definition of a function's prototype itself.