Answer:
A. Availability of capital for investment.
Step-by-step explanation:
It's the only answer that doesn't fit with the context, because the big industry was created after, it means that before Gilded Age, there isn't big industries and great investments.
After the explosive economic growth, principal companies donate big money to build hospitals, schools and social institutions for the people safety.
The growing supply of labor actually helped a lot. Back then, a lot of immigrants arrived the U.S., mainly from Europe. In addition to that, natural resources were an important part of this growth, water, steel and oil.