Final answer:
The infant mortality rate in the United States is closest to option a. 5 per 1,000 live births, based on available statistical data.
Step-by-step explanation:
The infant mortality rate (IMR) in the United States is a critical indicator of the health of the nation. Based on the information provided, while the IMR for high-income countries was about 6 per 1,000 live births in 2005, it's worth noting that the U.S., despite its extensive healthcare spending, has an IMR that is higher than in some other countries with lower medical costs.
However, more recent data indicates that the United States has seen improvements in its IMR. Statistics such as 6.17 deaths per 1,000 live births suggest that the accurate answer to the student's question would be closest to option a. 5 per 1,000 live births, as it is the option that is nearest to the statistics mentioned.