Now in the eighteenth year of king Jeroboam the son of Nebat reigned Abijam over Judah.
Three years reigned he in Jerusalem. And his mother's name was Maachah, the
daughter of Abishalom. (vv.1-2)
And in the twentieth year of Jeroboam king of Israel reigned Asa over
Judah. And forty and one years reigned he in Jerusalem. And his mother's name was
Maachah, the daughter of Abishalom. (vv.9-10)
The Harper Collins Study Bible note for 1 Kings 15.10: "Acording to this verse Asa and Abijam both had the same mother (cf. v. 2). This would make them brothers rather than father
and son (v. 8) Alternatively, it may be that Maacah held the important post of queen mother but was not necessarily Asa's biological mother."
It is possible, of course, that Abijam married his own mother (Maachah) and was, therefore, Asa's
father and brother.
The Biblically Correct answer: Abijam was Asa's father.
I'm going with Harper Collins on this one and say that the inspired author of 1 Kings messed up on verses 15:9-10. Asa's mom wasn't really Maachah.