Final answer:
Mary W. Dewson's main claim is that Social Security is essential for elderly assistance, anticipating its expansion to offer more coverage. The correct answer is option 4. The detail that best supports this is the statistic of elderly individuals living on less than $300 a year, highlighting the program's role in need-based assistance.
Step-by-step explanation:
In the passage from Mary W. Dewson's address, her main claim is that Social Security addresses a genuine need for elderly assistance, particularly for those without adequate resources to support themselves in old age. This program, she explains, is necessary because there is a segment of the population that cannot be insured privately for old-age dependency, akin to not being able to insure a car post-accident. Dewson anticipates that the Social Security program will continue to expand to cover more people over time.
The detail from the passage that best supports this main claim is her discussion about the significant number of elderly individuals living on less than $300 per year, which is considered below the minimum necessary for health and decency, further underscoring the importance of Social Security as a form of need-based assistance.