To answer this question, the user needs to know what a checkbox is and how it appears in a graphical user interface.

A checkbox is a user interface element that allows the user to select one or more options. The checkbox typically appears as a square box that can be checked or unchecked. When the box is checked, it indicates that the option is selected. When it is unchecked, the option is not selected.

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

A. button: This option is correct. A checkbox can also appear as a button, which allows the user to select an option by clicking on it.

B. RadioButton: This option is incorrect. A RadioButton is a different type of user interface element that allows the user to select only one option from a group of options.

C. ScrollBar: This option is incorrect. A ScrollBar is a user interface element that allows the user to scroll through a long list or document.

D. Both a and b: This option is incorrect because RadioButton is not a valid answer for this question.

E. All of the above: This option is incorrect because Checkbox does not appear as ScrollBar.

Therefore, the correct answer is:

The Answer is: A