To determine which event is activated when a RadioButton is selected, you need to understand the event model of the programming framework or language you are working with. In this case, assuming you are referring to a common event model, such as the one used in Microsoft's .NET framework, the answer would be B. CheckedChanged.
In most event-driven programming frameworks, a RadioButton control has a "Checked" property that represents its checked state. When the user selects or deselects a RadioButton, the Checked property changes, triggering the CheckedChanged event. This event is specifically designed to handle the change in the checked state of a RadioButton.