What is the best definition of an undocumented worker?

a) an immigrant who applies for entry and is admitted
b) an immigrant who lives and works in a country illegally
c) an immigrant who comes seeking educational opportunities
d) an immigrant who leaves an area to escape war or oppression

i know for a fact it is B.

B) an immigrant who lives and works in a country illegally.  

Explanation:

An undocumented worker is a foreign-born person who doesn´t have the legal permits needed to work in the country.

In the US, an undocumented worker is considered in violation of U.S. migration and naturalization law as well as not having permission to work in the United States, and can be deported.