The classic simplified charge structure of a mature thunderstorm is a vertical tripole: a weak 'screening layer' of positive charge at the very cloud top, a large main negative charge region in the mid-levels, and a smaller (but non-negligible in some storms) lower positive charge region near cloud base.
In this simplified three-box model, box A at the very top corresponds to the weak upper screening layer, which carries only a comparatively negligible positive...
Full option-by-option analysis available to enrolled students.