Final answer:
J.P. Morgan found his significant investment opportunity in the electric power industry, notably assisting in creating General Electric, and later consolidating the steel industry by creating U.S. Steel Corporation after buying out Carnegie.
Step-by-step explanation:
The idea that J.P. Morgan invested in, resulting in a massive footprint in American industrial development, was the electric power industry. He was particularly influential in the creation of General Electric. However, Morgan's most notable investment and greatest act of consolidation was in the steel industry when he bought out Andrew Carnegie to create U.S. Steel Corporation. Unlike his contemporary capitalists like Rockefeller, who was in the oil industry, or Carnegie, who developed the steel industry independently, Morgan was born into wealth and became a powerhouse in investment banking. His firm's significant influence extended into multiple sectors, playing a pivotal role in facilitating the growth of these industries.