Fron what I understood, it's a bit of process of elimination :
Since sp3 means only 4 electron arrows (sp2 means 3 electrons and so on), we know there are only 4 arrows. So the one with 6 arrows is out. Then, we know that the electrons need to fill the lowest energy first (lower ones) so we don't actually fill the anti sigma -that removes another one. Then between the last two, you know that the arrows need to have opposite spins. So the one with two arrows pointing up is incorrect. Therefore your answer must be D