Final answer:
Investing in skill development is the most reliable way to increase long-term earnings, as it leads to better job opportunities and economic mobility, despite initial sacrifices.
Step-by-step explanation:
For most individuals, investing in skill development is most likely to increase earnings over the long-term. Working additional jobs or longer hours can provide a temporary boost in income, but these strategies are not typically sustainable in the long-term nor do they significantly enhance an individual's earning potential. Entrepreneurship offers potential for substantial earnings but involves higher risk and is not a sure path to increased income for everyone. In contrast, investing in skill development, such as obtaining additional education or vocational training, can lead to higher-paying employment opportunities and is associated with long-term economic mobility. Despite the initial sacrifice and the deferral of income while pursuing education, this strategy can lead to a significant increase in personal wealth over the course of one's career.