Final answer:
To prevent students from modifying a worksheet in a shared folder, you should share the folder as view-only.
Step-by-step explanation:
If you don't want your students to modify a worksheet in a shared folder, you should share the folder as view-only. By doing this, your students will be able to view and access the contents of the worksheet, but they won't be able to make any changes to it. This ensures that the worksheet remains intact and unchanged.
Options A, B, and D are not suitable for achieving this goal. Asking students to use mobile devices does not prevent them from modifying the worksheet. Sharing the folder as view-only is the best solution to restrict editing abilities. Hangouts is a communication tool and not relevant to the goal of preventing modifications to the worksheet.