What additional marking should tanks bear?

The correct choice is that tanks should bear the marking instructing to keep 50 feet from buildings. This requirement is crucial for safety, as it helps prevent potential hazards associated with tanks, particularly those containing flammable or hazardous materials. Maintaining a safe distance from buildings minimizes the risk of fire or explosion in the event of a leak or rupture.

This marking is particularly important in urban environments where any accidents could involve nearby structures and people, leading to significant damage and injury. Establishing a clear distance helps ensure that emergency responders have enough space to safely manage any incidents involving these tanks, thereby enhancing public safety.

The other options do not address the specific safety concerns related to the placement of tanks in proximity to buildings, making them less relevant in the context of this question.