Final answer:
J.P. Morgan loaning $62 million in gold to the U.S. government best illustrates his success as a banker, showcasing his extraordinary financial capacity and influence.
Step-by-step explanation:
The detail that best shows that J.P. Morgan was a very successful banker is B. He loaned $62 million in gold to the US government. This action demonstrates Morgan's significant financial resources and influence, as it was highly unusual for an individual to provide such a substantial sum to bail out the federal government. Additionally, J.P. Morgan's involvement in other large-scale financial activities, such as controlling the finances of four of the nation's six largest railroads, and his powerful moves in the steel industry by financing and subsequently consolidating Carnegie's U.S. Steel, further exemplify his position as a successful and savvy banker.