Which of the following correctly defines a function that adds two numbers?

The chosen answer identifies that both provided options correctly define a function to add two numbers.

In the first option, the function is defined using an arrow function syntax. The line const add = (a, b) => a + b; captures the essence of JavaScript's modern syntax. This method is concise, using the arrow function to return the sum of a and b without needing the return keyword or curly braces, which is allowed when the function consists of a single expression.

The second option employs the traditional function declaration syntax: function add(a, b) { return a + b; }. Here, the function is named add, and it explicitly returns the sum of a and b. This style of function declaration is well-established and widely used in JavaScript.

Since both definitions correctly implement the intended functionality of adding two numbers, the selection of both as valid answers highlights the flexibility of JavaScript in defining functions using different syntaxes. Thus, the answer confirms a comprehensive understanding of proper function definitions in JavaScript, allowing for both modern and traditional coding styles.