An economic problem arises when there are scarcity and choices involved. This is because there are limited resources to meet unlimited wants and needs, and individuals, businesses, and governments must make choices about how to allocate those resources efficiently. Scarcity is a fundamental concept in economics, and it necessitates the need to make choices, trade-offs, and prioritize between various wants and needs. The other options mentioned in the question (few sellers, many buyers, and short supply of money) may be factors that influence market conditions or individual economic decisions, but they are not the fundamental cause of the economic problem.