What key should you hold down to place an object multiple times on the diagram?

To place an object multiple times on the diagram, the Alt key is the correct choice because it enables the feature of creating duplicates of the selected object when clicked repeatedly. This is particularly useful in diagramming as it allows for efficient replication of elements without needing to go through the full process of adding each individual object from the toolbox again. Holding down the Alt key while clicking allows users to quickly lay out multiple copies of the same element, enhancing productivity and design workflow.

Understanding this functionality is crucial in creating detailed and complex diagrams where the same components might need to be placed several times, making the process smoother and quicker. Different keys typically serve various functions in software applications, and recognizing the specific purpose of the Alt key in this context is important for effective usage of diagramming tools.