Final answer:
The US buys food at low costs to distribute to the poor primarily for poverty reduction, which is closely tied to achieving food security and improving the standard of living for those in need.
Step-by-step explanation:
When the cost of food is very low and the US buys food with the intent of distributing it to the poor, the primary reason for this policy is C. Poverty reduction. This action is taken to ensure that those with the least resources have access to basic nutrition and sustenance, directly targeting the issue of food insecurity. Nobel Prize-winning economist Amartya Sen noted that macroeconomic policies play a vital role in distribution and that by striving for stable inflation, full employment, and the education of women, countries can promote a more even distribution of food and reduce poverty levels. This strategy aligns with the broader goal of ensuring food security and improving the standard of living for the impoverished population.