To solve this question, the user needs to be familiar with SAP programming and the different components of SAP GUI.

The AT USER-COMMAND event is used in SAP programming to handle user input for custom toolbar buttons. When a user clicks on a custom toolbar button, the AT USER-COMMAND event is triggered, and the corresponding function code is executed.

Now, let's go through each option and explain why it is right or wrong:

A. Screen painter: This option is incorrect because the screen painter is used to design the layout and appearance of SAP GUI screens. It does not define the functions that are executed when a user clicks on a custom toolbar button.

B. ABAP report: This option is incorrect because ABAP reports are used to generate reports or lists of data in SAP. They do not define the functions that are executed when a user clicks on a custom toolbar button.

C. Menu painter status: This option is incorrect because the menu painter is used to design the menus and menu bars in SAP GUI screens. It does not define the functions that are executed when a user clicks on a custom toolbar button.

D. ABAP Dictionary: This option is incorrect because the ABAP Dictionary is used to define and manage database objects in SAP. It does not define the functions that are executed when a user clicks on a custom toolbar button.

Therefore, the correct answer is option C. The functions that are executed when a user clicks on a custom toolbar button are defined in the Menu painter status.