Which statement accurately describes the composition of structural lightweight concrete?

The correct choice accurately captures the composition of structural lightweight concrete, which is made using Portland cement and water in combination with lightweight aggregates such as shale, clay, or slate. These materials contribute to the unique properties of lightweight concrete, including reduced density and improved insulation capabilities. The incorporation of these aggregates allows for a structure that is lighter than traditional concrete, which can be beneficial in various construction applications, especially where reducing the load on supporting structures is essential.

The other options fail to encompass the whole picture of structural lightweight concrete's composition. Sole reliance on sand and water does not account for the necessary binding agent (Portland cement) or any aggregates. Similarly, focusing solely on organic materials overlooks the fundamental role played by cement and non-organic aggregates in creating the necessary strength and structural integrity. Lastly, while air is an important aspect of lightweight concrete's formation, claiming it is mainly composed of air and water disregards the essential role of the cement and aggregates.