You use the . . . . . . . . constant to include the Exclamation icon in a message box.

The correct answer is C. MessageBoxIcon.Exclamation.

The MessageBoxIcon.Exclamation constant is used to include the Exclamation icon in a message box. The Exclamation icon is a yellow triangle with an exclamation point in the center. It is used to indicate a warning or caution.

The other options are incorrect. MessageBox.exclamation is not a valid constant. MessageBox.Iconexclamation is a typo. MessageBoxWarning.Icon is a valid constant, but it does not include the Exclamation icon.