Under normal weather and sea conditions when securing a stack of containers with twist locks, lashings are required when the tier exceeds what height?

Correct Answer: Option D — Two containers

Standard container securing manuals and cargo stowage guidelines stipulate that tiers up to two high may utilize automatic or manual twist locks without additional vertical lashings under normal weather conditions. Once the stack height exceeds two containers, exterior lashings such as rods and turnbuckles become mandatory.