Final answer:
The Jiang family released Song Po-po because they no longer needed her services (option D), as indicated by the lack of evidence for other reasons such as retirement, a new job, or illness.
Step-by-step explanation:
The Jiang family finally released Song Po-po from her job of housekeeper because they didn't need her services anymore. This is likely the case as the passages provided do not mention her wishing to retire, finding a better-paying job, or being unable to work due to illness.
The logical conclusion, considering the lack of other contextually supported reasons, is that the family's circumstances changed, thus negating the need for Song Po-po's employment. This can happen in households when children grow up or when the financial situation alters, leading to a scaling back on domestic help.