How do you declare a constant variable in JavaScript?

Declaring a constant variable in JavaScript is done using the const keyword, which is key to ensuring that the variable's value cannot be reassigned once it has been initialized. When you use const variableName = value;, you create a variable that must be assigned a value at the time of declaration, and attempts to reassign it later in the code will result in an error. This is particularly useful for maintaining immutability in your code and preventing accidental changes to important data.

The let and var keywords are used for declaring variables that can be reassigned. let allows for block-scoped variables, while var declares variables that are function-scoped or globally scoped but can also be reassigned. The option that uses the word 'constant' is not valid syntax in JavaScript, as it doesn't recognize 'constant' as a keyword for variable declarations. Therefore, using const is the appropriate and correct way to declare a constant variable in JavaScript.