Which one of the following Excel actions uses an open Excel file instead of launching Excel?

The action that utilizes an open Excel file rather than launching a new instance of Excel is "Attach to running Excel." This option allows you to connect to an existing Excel application that is already open. By attaching to a running instance, you can perform various operations on the already opened workbooks, which can enhance efficiency, especially when dealing with multiple files that might already be in use.

In contrast, launching Excel initiates a new instance of the application, separating it from any existing workbook sessions. The other actions, such as setting an active worksheet or activating a cell, generally occur after you have established a connection to Excel, either by launching a new instance or attaching to an already running one. However, those actions themselves do not directly address whether a new instance is created or an existing one is used.