What percentage of the carbon atoms 2nd energy level is filled?

The second energy level can hold total of 8 electrons.

Carbon has atomic number equal to 6. There are 2 electrons in the first energy level and rest of the 4 electrons are in the second energy level.

So only 4 are present in second energy level while it can hold total 8.

4 is half of 8. So the percentage of energy level filled is half of 100%, the answer is 50%.

50%