If you are unsure about whether or not to include a citation because you have seen that information somewhere before,

a. It means the information is not important enough to cite

c. You should consider it common knowledge

b. You should include a citation anyway

d. You should leave it out of your paper

The answer is letter B.

You should include a citation if you seem to have seen that information before. But in citing the source, you should choose the most closely related information to the material you are writing about in your paper. Others may consider this a common knowledge but as a precaution, you should always cite your materials.