What probably caused the crumbling of Cleopatra's Needle, and how can further damage be prevented?

The correct choice identifies traffic vibrations as a significant factor contributing to the crumbling of Cleopatra's Needle. Over time, the vibrations from heavy vehicle traffic can wear down the structural integrity of historical monuments, leading to deterioration. By mitigating such traffic impacts, for example through strategies to reduce congestion or limit heavy vehicle access near the monument, the stress on the structure can be decreased.

While weather exposure, pollution, and earthquake activity are also valid concerns for preserving ancient monuments, in the context of Cleopatra's Needle, traffic-related vibrations present a more immediate threat, making it essential to prioritize mitigating these effects for the preservation of the monument. Weatherproofing, relocation, and foundation reinforcement, while important considerations for other structures, may not directly address the specific issue highlighted by traffic vibrations in this scenario.