Which of the following is a valid use of the 'let' statement in JavaScript?

The 'let' statement in JavaScript is used to declare a block-scoped variable. In this context, the option that uses 'let x = 5;' demonstrates the correct syntax for declaring a variable named 'x' and assigning it the value of 5. This is the standard way to create a variable using 'let,' which allows for its scope to be confined to the block in which it is defined.

Using 'let' in this manner helps to maintain clean and predictable variable scope, making it easier to manage variable declarations within loops and conditionals compared to using 'var.' The declaration is straightforward and follows the correct syntax rules that JavaScript enforces for variable declarations.

The other options illustrate incorrect applications of 'let.' For example, attempting to assign a value to a number or using the arrow function syntax with 'let' does not adhere to valid declaration conventions in JavaScript. Thus, the option 'let x = 5;' stands out as the only valid use of the 'let' statement.