A user is inserting a 3-D reference into a formula. After activating the first worksheet, which key must the user hold while activating the last worksheet?

I'd say the shift key.

You can quickly select one or multiple sheets by simply clicking the tabs of sheets at the bottom of the Excel window. To select two or more adjacent sheets and activate them, you are required to click the tab for the first sheet, then hold down SHIFT key while you click the tab for the last sheet.