What is an indicator of when soil is at its optimum moisture content?

Answer: B — The soil can be compacted to its greatest density

Optimum moisture content is defined as the amount of water in the soil that allows it to be compacted to its greatest density. This is critical in construction and highway engineering because achieving maximum density is essential for the stability and durability of the structures being built. At this optimum point, the soil particles are arranged in a configuration that allows for maximal inter-particle contact and friction without excess water that would lead to a reduction in strength and stability. When the moisture content is just right, it ensures that the soil has enough water to lubricate the particles, facilitating compaction, but not so much that the soil becomes overly saturated and loses its structural integrity. The other options indicate states of the soil that do not represent optimum moisture content. Dry and crumbly soil often indicates insufficient moisture, while very wet and sticky soil suggests that the moisture content is above optimum, which can hinder compaction and lead to issues such as deformation. Pooling water is a sign of excessive moisture, which is counterproductive to achieving optimal soil conditions for construction purposes. Thus, the correct answer accurately reflects the conditions needed for ideal compaction in soil.