Which type of block does not require formwork for solid concrete foundation walls but requires reinforcement?

The correct choice is concrete blocks because they are designed to be stacked without the need for additional formwork when building solid walls. Concrete masonry units (CMUs) are commonly used in construction for foundation walls due to their structural integrity, versatility, and ease of handling.

When constructing walls with concrete blocks, especially for foundations, it's essential to incorporate proper reinforcement. This is typically done by using rebar or reinforced concrete ties, which help to enhance the wall's stability and resistance to lateral loads, such as soil pressure. The use of concrete blocks allows for quicker construction processes as they can be laid directly without requiring formwork that would be necessary for poured concrete.

While bricks, stone blocks, and poured concrete can also be used in construction, they generally involve different installation requirements. For example, brick walls typically require mortar to bond the bricks together and wouldn't be assembled in the same way as concrete blocks. Stone blocks can be irregularly shaped and may need specific techniques for placement and securing. Poured concrete, on the other hand, relies entirely on formwork for shaping, since it is a liquid mix when placed. Hence, concrete blocks stand out as the preferable option for building solid foundation walls without the need for a supporting mold.