Final answer:
When the U.S. flag is displayed on a staff projecting from a building, the union or blue field should be on the left side. This makes the position of the union prominent as per flag etiquette and symbolic of the flag's advance.
Step-by-step explanation:
When the U.S. flag is displayed from a staff projecting horizontally or at an angle from the windowsill, balcony, or front of a building, the union (or blue field) of the flag should be placed on the left side. The correct answer is A) On the left side.
This orientation ensures that the union is in a position of prominence, which is consistent with flag etiquette. It appears to viewers as if the flagpole were transparent, displaying the flag's natural orientation with the union to the viewer's left when facing the building.
Moreover, the placement of the union on the left side symbolizes the flag's advance and is never dipped to the ground as a sign of respect.