The shortcut key for 'adding background colour' is
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Alt + Backspace
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Shift + Backspace
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Ctrl + Backspace
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Ctrl + Alt
A
Correct answer
Explanation
In many image editing programs, Alt + Backspace fills the current selection or layer with the foreground color, while Ctrl + Backspace fills it with the background color. Given the options, Alt + Backspace is the standard choice for filling.