A form’s . . . . . . . . event is triggered when you click the Close button on its title bar.

The correct answer is D. FormClosing.

The FormClosing event is triggered when you click the Close button on a form’s title bar. This event gives you the opportunity to perform any necessary cleanup before the form closes. For example, you might want to save the user’s data or close any open files.

The Close event is triggered after the FormClosing event has been processed. This event is too late to perform any cleanup, so it is typically used to close the form itself.

The CloseForm event is not used in Windows Forms programming. It is used in ActiveX programming to close a form that is hosted in another application.

The Form.Close() method is used to close a form. This method can be called from code or from the form’s menu. When the Form.Close() method is called, the FormClosing event is raised. The FormClosing event handler can cancel the close operation by setting the CancelEventArgs.Cancel property to true.

Here is an example of how to use the FormClosing event:

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
if (MessageBox.Show("Are you sure you want to close the form?", "Close Form", MessageBoxButtons.YesNo) == DialogResult.No)
{
e.Cancel = true;
}
}

In this example, the FormClosing event handler displays a message box asking the user if they are sure they want to close the form. If the user clicks No, the CancelEventArgs.Cancel property is set to true, which cancels the close operation.