Which of the following statements correctly defines a variable in Kotlin?

The statement "var age = 20" is the correct way to define a variable in Kotlin. In Kotlin, the keyword "var" is used to declare mutable variables, meaning that the value of the variable can be changed after it has been initialized. In this case, "age" is initialized to the integer value of 20, and you can later modify the value by assigning a new integer to "age," for example, "age = 25".

The other options do not conform to Kotlin's syntax for variable declaration. Using "let" is not a declaration keyword for creating variables; instead, it is a standard library function that operates on values or scopes. The keyword "const" is used for compile-time constants in Kotlin, but it must be used within an object or a top-level declaration and always requires the type to be defined. Finally, "def" is not a recognized keyword in Kotlin for variable declarations; it is typically found in other programming languages like Python. Therefore, only "var age = 20" adheres to Kotlin's variable declaration rules.