Option D is correct because the textbook explicitly states that lapies are formed when most of the limestone surface is eaten away by pits and trenches, leaving it 'extremely irregular with a maze of points, grooves and ridges or lapies'. These form due to differential solution activity along parallel to sub-parallel joints. Option A describes swallow holes (small to medium sized shallow depressions). Option B describes sinkholes, which have 'an opening more or less circular at the top and funnel-shaped towards the bottom'. Option C is not the defining characteristic mentioned in the text.