if you are applying for citizenship, you might come across this question on your n400 form on Part 2. Information About Your Eligibility
you have these choices:
A. I have been a lawful permanent resident of the United States for at least five years

B. I have been a lawful permanent resident of the United States for at least five years, and I have been married to and living with the same U.S. Citizen for the last three years, and my spouse has been a U.S. Citizen for the last three years

C. I am applying on the basis of qualifying military service

D. Other (Explain)

The most common choice is A, but Also B

so what is the difference between A and B

well, A is for people who have a green card but they are not married to a U.S Citizen. For example, you got your green card from your parents. in this case, you can apply for citizenship after 5 years of being a Permanent Resident.

B is for people who got their green card by getting married to a US citizen. if you have been married to a US citizen, you can apply for citizenship after 3 years.