Which of the following is a valid JavaScript variable declaration?

The correct choice for a valid JavaScript variable declaration is one that adheres to the syntax and conventions of JavaScript. Both the option that uses 'let' and the one that uses 'var' are valid declarations.

In JavaScript, 'let' and 'var' are keywords used to declare variables, with 'let' allowing for block scope while 'var' is function-scoped. Thus, 'let myVariable;' and 'var myVariable;' are both acceptable forms of declaring a variable. The keyword 'define' is not recognized in JavaScript as a variable declaration method, and 'declare' is also not a valid keyword for variable declaration. Hence, options involving 'define' and 'declare' are not correct.

Using 'var myVariable;' creates a variable named 'myVariable' that is accessible throughout the function in which it is declared, or globally if declared outside of any function. This is foundational knowledge for anyone working with JavaScript, as these declaration types determine how variables behave in different scopes throughout the application.