Correct Answer: DThe most likely cause of the printer mistakenly adding white space in the middle of the page when printing A4-sized labels is incorrect page orientation. Page orientation settings determine whether the document is printed in portrait or landscape mode. If the orientation is incorrectly set in the printer settings or software, it can result in misaligned printing, leaving unexpected white space, particularly in the middle of the labels. Other issues like a contaminated fuser, worn rollers, or a misfeed would typically cause different types of print defects such as smudges, streaks, jams, or misalignment but not specifically white space in the middle of the page.