checkboxes - Select multiple options or only one exclusive

I want to give the user a list of possible options to select with checkboxes. But what if I want him to provide a way to select only one without having to deselect the others?

Eg:

X  OptionA
X  OptionB
   OptionC
X  OptionD

I'm thinking something like "shift-click". If he shift-clicks OptionC, the other 3 automatically deselect.

Answer

Don't hide functionality in shortcuts.

If your user's work flow demands the need to quickly and easily deselect all options, make the option explicit. Provide a button that does what it says:

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Now it is clear to the user how to deselect everything they've selected. A click of the button and a click of "Option 3" gives them what they want. Easily.

Don't confuse system shortcuts

Shift-click has meaning in many systems and when dealing with lists. It will, often, select (or de-select) all items between the last selection and the item just clicked. Don't redefine a user's mental model of how that shortcut works.