This is an example of a strengthen question. Therefore, we are looking for an answer which most helps to explain why unemployed retirees are more likely to have a heart attack on Monday based on the information provided in the passage.
You have correctly identified that C through E are incorrect. Nice work!
As for the distinction between A and B, the key point to focus on in this question is that we are looking at "unemployed retirees." By specifying this subset of the population, retirees with part-time jobs are eliminated as they are employed.
A would, therefore, be the correct answer here because it provides a source of stress on Mondays for retirees who do not have jobs.
Hope this helps! Please let us know if you have any further questions.