To select multiple objects for modification, which key must be pressed?

To select multiple objects for modification in many software applications, the Ctrl key is the correct choice. Pressing the Ctrl key while clicking on individual objects allows users to select them one at a time without deselecting previously selected items. This method is especially useful for selecting non-contiguous objects, enabling modifications to a group of selected items collectively, rather than individually.

The other keys mentioned serve different purposes. For instance, the Shift key is typically used to select a range of objects that are contiguous, not to individually select separate items. The Alt key is often used for alternative actions or to modify behaviors in certain contexts, but it is not standard for multi-selecting items. The Esc key is used to cancel or exit out of current selections or operations, rather than facilitating the selection process. Thus, using the Ctrl key is essential for effective multiple selections when working in environments focused on object manipulation.