The blackboard should make the key ideas of a lesson easy for students to follow.
Why D fits: Writing important points clearly, with legible letters and suitable spacing, supports understanding and note-taking.
Why the others are weaker: Writing quickly can reduce readability; filling the board before discussion can reduce interaction. Student answers may be written on the board, but that is not the general rule asked here.
Practical point: Organise the board in a logical sequence instead of copying everything said during the lesson.