To which pair of taxpayers do the rules for children of divorced or separated parents apply?

A father ($6,149 AGI) and his girlfriend ($40,643 AGI) who have never married but who have lived together for three years. The qualifying child who lives in the household is the child of only the father.

A father and mother who have never married and have not lived together for the last six months of the year, whose child is their qualifying relative, not a qualifying child.

A grandmother and grandfather who are divorced, whose grandchild is the custodial grandparent's qualifying child.

A father and mother who are still married, but who have not lived together for the last six months of the year, whose child is the custodial parent's qualifying child.