Answer
Choice A.
Work Step by Step
All parts of the dancer are in free fall, and have the same downward acceleration.
Her head and her neck have the same downward acceleration, produced by gravity. Her neck does not exert any upward supporting force on her head.
In other words, the weight of her head is mg, and it is accelerating with magnitude g, so mg is the net force. The neck exerts zero force on her head.
