Answer: A
The geometric shape used by David Smith in his sculpture Cubi XIX is A. Cuboids.
Cuboids are three-dimensional shapes that have six rectangular faces, with each face meeting at right angles. They resemble rectangular prisms. David Smith's Cubi XIX sculpture is part of his Cubi series, which consists of abstract geometric forms made primarily of welded steel. The sculptures in this series often feature stacked or interlocking cuboid shapes, showcasing Smith's exploration of form, balance, and space.