What feature is necessary for lumber stacking supports?

The necessity for lumber stacking supports to be level and solid is essential for maintaining the structural integrity and safety of the stack. A level surface ensures that the weight of the lumber is evenly distributed, preventing any potential tipping or shifting that could lead to accidents or damage. A solid foundation is critical to prevent sinking or settling over time, which could destabilize the stack and create hazards for workers or equipment in the vicinity. This level and solid base can help to minimize risks associated with poor stacking practices, such as lumber falling or collapsing, which can pose serious safety concerns on a job site.

The other options, such as being on sand, on uneven terrain, or on a slope, introduce instability and uneven weight distribution, which are not suitable conditions for safe lumber stacking.