What does the following code do: `int x = 5; x += 3;`?

The code int x = 5; x += 3; performs an operation where the initial value of x is set to 5. The statement x += 3; is shorthand for adding 3 to the current value of x and then assigning the result back to x.

Initially, x is 5. When you execute x += 3;, it computes 5 + 3, resulting in 8, and then updates x to hold this new value. Thus, after the execution of the code, the value of x becomes 8, which is why the correct answer is that it adds 3 to x, so x becomes 8.

The other options imply incorrect operations: setting x to 3 or subtracting values, neither of which accurately represents the operation being performed in the code snippet.