A family of Microsoft word processing software products for creating web, email, and print documents.
This is far from a simple task in Word. Please read the thread at https://answers.microsoft.com/en-us/office/forum/office_2010-word/multiple-selection-from-a-drop-down-menu-in-ms, in which I've written several detailed replies. If you still need more guidance after reading that, please post back here with more details about what you want to do and/or what parts of the explanations are giving you trouble.