Each button has same height and width because they are . . . . . . . . the interface.

The correct answer is: C. Clustered.

Buttons are clustered when they are placed close together in a group. This can be done to make them easier to find and use, or to create a visual effect.

• Stacked buttons are placed one on top of the other. This can be done to save space, or to create a sense of hierarchy.
• Queued buttons are placed in a line, one after the other. This can be done to indicate that the buttons should be used in a specific order, or to create a sense of movement.
• Heaped buttons are placed randomly, without any apparent order. This can be done to create a sense of chaos or excitement, or to make the buttons more difficult to find.

In the case of the question, the buttons are clustered because they are placed close together in a group. This is likely done to make them easier to find and use.