Which command allows you to combine multiple objects into a single object?

The UNION command in AutoCAD is specifically designed to combine multiple 2D or 3D objects into a single object. When using this command, you can select two or more shapes, such as circles, rectangles, or 3D solids, and the resulting object will encompass the area of all the selected entities. This is particularly useful in operations involving solid modeling where you want to create a more complex shape or a single solid body from simpler components.

In contrast, while similar commands like JOIN can be used to connect objects in a specific way, it generally pertains to joining lines and polylines into a single polyline rather than combining different types of objects. The COMBINE command does not exist in standard AutoCAD terminology, and MERGE is not a recognized command in the context of combining objects. Therefore, UNION is distinctly suited for the purpose of creating a unified object from multiple selections, making it the correct choice in this scenario.