D. It should be never flown at half-mast.

The Royal Standard of the United Kingdom is indeed the personal flag of the British monarch, and it is traditionally flown to indicate the sovereign's presence at a particular location. However, contrary to option D, there are instances where the Royal Standard can be flown at half-mast. This usually occurs as a sign of mourning when a member of the royal family or another significant individual dies. The half-mast position indicates respect for the deceased. Therefore, option D is not true about the Royal Standard.