3D Shapes:
A 3D shape, also known as a three-dimensional shape, is a solid object that has three dimensions - length, width, and height. These shapes can be measured in three directions and have volume.

Cube:
A cube is a specific type of 3D shape that has six square faces, all of which are equal in size. It is a regular polyhedron, meaning that all its faces are congruent and its edges are of equal length. A cube has 12 edges and 8 vertices.

Square:
A square is a 2D shape that has four equal sides and four right angles. Unlike a cube, it does not have height or depth. It lies completely on one plane and is considered a 2D figure.

Triangle:
A triangle is also a 2D shape that has three sides and three angles. It is a polygon with three vertices and three line segments connecting them. Like a square, a triangle is a 2D figure and does not have height or depth.

Hexagon:
A hexagon is a 2D shape that has six sides and six angles. It is a polygon with six vertices and six line segments connecting them. Similar to a square and a triangle, a hexagon is a 2D figure and does not have height or depth.

Conclusion:
Among the given options, the cube is the only shape that is three-dimensional. It has length, width, and height, making it a solid object. The other options - square, triangle, and hexagon - are all two-dimensional shapes that lie on a plane and do not have depth. Therefore, the correct answer is option 'A' - Cube.