In Access to open an existing database press

To open an existing database in Microsoft Access, you can use the keyboard shortcut CTRL+O. This combination of keys opens the Open dialog where you can browse and select the database file you want to open. The CTRL+N shortcut is used to create a new, blank database instead of opening an existing one. ALT+F4 is a general Windows shortcut to close the currently active window, and it is not used to open databases. Therefore, the correct option is Option B: CTRL+O.