If a "plane slices horizontally" through a solid cube, the shape of the "cross-section" would be a square.
<u>Explanation:</u>
A solid cube is a block with six distinct faces. Every vertex angle of a cube is exactly 90°.
If the cube is placed on one of its horizontal faces and is sliced horizontally keeping the cut parallel to its horizontal face, the sliced cube would get the shape of a square("if viewed from the top").