The function that displays a pop-up window in VBA is the MsgBox function. This function is used to display a message in a dialog box and can include buttons for the user to click.

Option A) MsgBox - This option is correct because this function displays a pop-up window. Option B) InputBox - This option is incorrect because this function displays an input box to prompt the user for input. Option C) TextBox - This option is incorrect because this is a control used to display or enter text in a user form. Option D) Both a and b - This option is incorrect because option B is incorrect. Option E) All of the above - This option is incorrect because option C is incorrect.

Therefore, the correct answer is option A, MsgBox.