Final answer:
Marcia's follow-up study found that some adolescents reverted from a stable identity achievement to earlier stages such as foreclosure or diffusion, suggesting that identity formation can be fluid and subject to change.
Step-by-step explanation:
In adolescence, the development of a personal identity is crucial, as theorized by Erikson. Marcia, building upon Erikson's theories, studied identity achievement in adolescents. His findings challenge the notion that once a stable identity is reached, it is maintained consistently throughout life. Marcia's follow-up study indicated that some adolescents who had achieved a sense of identity had regressed to an earlier stage, either to the foreclosure or diffusion status. This shows that identity status is not always a linear progression and can be fluid, changing based on new experiences and challenges.
The correct answer to the original question is (a) Some adolescents who had achieved a sense of identity had returned to the foreclosure or diffusion status.