Frequently used commands can be placed in or on the in the top left corner of the window.
select one: a. taskbar b. quick access toolbar c. commands tab d. ribbon

b. Quick Access Toolbar                                        

Explanation:

Quick Access Toolbar is a customizable toolbar that is displayed in Microsoft Excel, Microsoft Word, and other Office products.It appears in the top left corner of the window by default.  QAT provides with quick access to frequently used features e.g. the Save, Undo, Redo features.When you click on the drop down menu of QAT, it allows you to customize QAT. This enables you to add and remove the commands that are displayed in the toolbar. These are default commands however, it allows you to add your own commands. You can add commands to the QAT that you frequently use. You can import or export these modifications.For example Excel Quick Access Toolbar contains only 3 buttons by default Save, Undo and Redo.