Hello there!
Your question asks which distribution system shipped more than 90% of the U.S coal.
Answer: D). Railroads
The reason why answer choice "D). Railroads" is the correct answer is because this was the distribution method that was used when shipping coal around the U.S.
Railroads were a very reliable source of distribution/transportation/and etc. in the United States. Railroads were especially impactful when it came to distributing things around the country. Railroads were quick, could hold a lot of things, and get their safely. That's why businesses who sold coal used railroads since they could get a lot across the country quickly and safely.
Trains on the railroads could hold a lot of things, and businesses sold coal in huge volumes, in which they needed something to transport the huge volume of coal, and that's where the rail roads came in handy. If they didn't use railroads, then there would be a lack of sales since they can't get their products across the country in a timely manner.
I hope this helps!
Best regards, MasterInvestor