The string “12” has been written into the str variable: (let str = “12”;). Then the operations str = +str is performed. As a result, the variable str will contain:

The variable str will contain the number 12 as a result of the operation str = +str.

Let’s break down the code step by step:

1. Initially, the string “12” is assigned to the variable str using let str = "12";.
2. The unary plus operator (+) is used to convert the string value to a number. When applied to a string, the unary plus operator attempts to convert the string to a numeric value.
3. In this case, the unary plus operator is applied to the string “12”, resulting in the number 12.
4. The result of the conversion is then assigned back to the variable str using str = +str. Since the conversion is successful, the variable str now holds the number 12.

To illustrate this in code:

let str = "12";
str = +str;
console.log(str); // Output: 12

After executing the code snippet, the variable str will contain the number 12.

So, the correct answer is 12.