In the add function, which usage of x would cause a scope error?

The usage of x that would cause a scope error occurs in the context where x is being referenced without being defined in the accessible scope. In the provided scenario, if the variable x has not been declared or is not in scope when the add function attempts to access it, attempting to evaluate the expression "var b = x + y;" will result in a reference error.

For option D, if x is not defined before this expression is executed, JavaScript's engine doesn't recognize x, leading to a scope error. In contrast, other options either declare variables or use them in a manner where they wouldn’t lead to a scope error as long as y is defined in the same scope or a higher scope. Thus, failing to have x defined is what specifically triggers the scope error in this case.