Why is it important to use a nested loop to iterate over all of the elements in a $2$D list?

A nested loop is required because the first loop iterates over the inner lists and the second loop iterates over the elements in each inner loop.

The loops do not need to be nested. Two consecutive for loops do the same job as the nested loop.

Using a single loop on a $2$D list will cause an error.

It is not important. A traditional loop is just as good as a nested loop.