Final answer:
For 18 to 32-year-olds, financial stress is likely the most commonly reported source, particularly in light of the 2008-2009 economic downturn and the pressures of increased college tuition and debt.
Step-by-step explanation:
The source of stress most commonly reported by 18 to 32-year-olds tends to vary but based on the provided texts and the demographic impacts of external factors such as economic downturns, potential stressors for this age group include financial stress, academic stress, and relationship stress. The 2008-2009 economic downturn highlighted financial stress due to job security and loss of savings, especially among college-educated men. At the same time, college students today face academic stress due to increased tuition and burdensome debt, while stress in relationships can arise from disagreements or lack of emotional support.
Though not explicitly stated, it can be deduced that financial stress might be the most common source, considering the broader implications it has on various aspects of life and well-being. This is supported by the introduction detailing the pressure on today's college students stemming from financial concerns.