A lever has a total length of 12 meters with a fulcrum at the exact center of the lever. Which best describes how to double the mechanical advantage of the lever?

So lever and fulcrum are one of the basic or simple machines that could make the work easier and faster. So in your example, to double its mechanical advantage of its lever, the the length of the handle of its lever should be more than the length where the load is place, in that way you will double its advantage.