Answer: The correct answer is A: vertical mobility.
Explanation: When a move involves a change in social class, it is called “vertical mobility” and involves either “upward mobility” or “downward mobility. In this specific case, Becky's story ilustrates downwards mobility because she lowered her previous social status. For it to have been horizontal, her status should have remained the same though her home or job would have changed. For it to had been structural, the change should have involved a larger group of people of the society, and not just an individual.