The proper intermediate steps here would be 15,10,20,18 to 15,10,18,20 to 10,15,18,20, or answer A. Bubble sorting involves switching elements around if they are in the wrong order. It starts at the top and works its way down.
So, for this particular set of numbers, it’s probably one of the best things to use since the list is so short.
Bubble sorting is not used all that often because with longer lists, it takes forever to switch everything so that it is properly sorted from top to bottom.
Especially if one of the larger items is already at the bottom. Even the list of eight that acts as an example on Wikipedia takes about five minutes to be sorted out this way.