Used primarily in the united states for electricity generation: 2. remains of ancient organisms, modified underground for long periods by temperature and pressure: 3. the world's most abundant fossil fuel: 4. a mixture of hundreds or thousands of different hydrocarbon molecules: 5. primary fuel used in the united states for space and water heating: 6. created very slowly and considered nonrenewable at current extraction rates: 7. used primarily in the united states as a source of vehicle fuels: 8. produces the least carbon dioxide per unit energy when combusted:

1. Used primarily in the United States for electricity generation = coal
In America, coal is mostly used for creating electricity all over the continent. However, this doesn't only refer to the US - coal-fired plants actually fuel 41% of world electricity, so it is usually the preferred choice because of the price and benefits it gives.

2. remains of ancient organisms, modified underground for long periods by temperature and pressure = coal, oil, and natural gas
These fossil fuels can remain underground for very long periods of time, where they will be modified into something else. They have originated from ancient organisms, and they represent their remains from ancient times.

3. the world's most abundant fossil fuel = coal
As I've previously stated in the first question, coal almost comprises half of global electricity generation, amounting to 41% of global consumption. Whereas other sources of electricity are also used, coal is by far the most popular choice.

4. a mixture of hundreds of thousands of different hydrocarbon molecules = oil
Oil is a chemical substance which can't be mixed with water, and as your definition says, it is hydrocarbon, meaning it has high contents of hydrogen as well as carbon. It is used for many things, and its price is always high.

5. primary fuel used in the United States for space and water heating = natural gas
Natural gas is the preferred choice in the United States when it comes to heating water as well as space. As the name itself says, natural gas occurs naturally, meaning it can be found in nature, consisting primarily of methane.

6. created very slowly and considered nonrenewable at current extraction rates = coal, oil, and natural gas
Nonrenewable means that it cannot be created naturally all over again. This also means that people have to be careful when it comes to nonrenewable resources given that once they are depleted, we will be left without them possibly forever, or until we figure out how to replicate them artificially

7. used primarily in the United States as a source of vehicle fuels = oil
Oil is the preferred choice in America when it comes to fueling vehicles, such as gasoline and diesel. Although there are other choices such as propane, compressed natural gas, hydrogen, electric batteries, etc., oil remains a favorite.

8. produces the least carbon dioxide per unit energy when combusted = natural gas
Unlike other gases, when natural gas is combusted, it doesn't produce as much carbon dioxide per unit energy as other gases do. This is why it is a preferred choice for heating spaces as well as water because it is good for the environment.