A“resident alien” of the united states, refers to someone who
a.
is an immigrant living in the country illegally.
b.
was born in the united states.
c.
was born in another country but has been naturalized.
d.
is a citizen of another country legally residing in the united states.

Correct  d. is a citizen of another country legally residing in the United States.

Explanation:

If you think about the term, "resident alien," the answer is essentially spelled out by that term.  The person is an "alien," which means someone from another country (not a US citizen).  At the same time, the person is a "resident," which means the individual is residing in this country.

If an immigrant is living in the country illegally, we call that person an "illegal immigrant" (just like the description).

If a person was born in the United States, that person is a US citizen.

Also, someone born in another country that has been naturalized is also a US citizen.

i believe your answer is d)