a) dragover
b) dragenter
c) dragstart
d) dragleave

Answer: a
Explanation: The dragover event is fired as the mouse is moving over an element when a drag is occurring. Much of the time, the operation that occurs during a listener will be the same as the dragenter event. When draggable element enters a drop target then dragenter event has occurred. A dragleave event occurs when it leaves the valid drop target. Whenever a user starts to drag an element, dragstart is fired.