Masonry units can be:

Masonry units can indeed be both hollow and solid, which makes the correct answer the combination of the two. Hollow masonry units typically feature voids or cavities within their structure, which can significantly reduce their weight and increase insulation properties while maintaining strength. These types of units are often used in construction for walls where reduced weight may be beneficial, or where increased thermal performance is desired.

On the other hand, solid masonry units are made without any voids, providing greater mass and strength. They can be used in applications that require load-bearing capabilities or where stability against structural stresses is essential.

The versatility in masonry unit design allows builders and architects to choose between hollow and solid units based on their specific project needs, including design requirements, cost considerations, and the desired properties of the walls being constructed. This is why the correct answer encompasses both hollow and solid units.