Which of the following is NOT a suitable shelter from a nuclear blast?

The open rooftops are not considered a suitable shelter from a nuclear blast due to their exposure to direct fallout, heat, and radiation. In the event of a nuclear explosion, the principle of shielding is critical to reduce exposure to harmful radiations and blast effects. Open rooftops do not provide any substantial barrier to prevent the wave of heat and the lethal radiation that accompanies a nuclear detonation.

In contrast, other options like deep foxholes, hills, and ditches provide a degree of protective cover. Deep foxholes offer significant protection from blast effects and radiation due to their depth and the surrounding earth. Similarly, hills can shield individuals from the direction of the blast and fallout, while ditches, although less effective than foxholes, still provide some cover from radiation and debris. These forms of shelter minimize the potential for injury or exposure during a nuclear event, making them far more suitable than open rooftops.