Answer
a. AB$^-$
Work Step by Step
It is impossible for a type O$^+$ baby to have a type AB$^-$ mother.
You can think of the type O blood type as being OO, one O from the mother and one O from the father (O is also recessive, so an AO blood type is read as A) .
A type AB$^-$ mother is type AB and is unable to provide any O's to the baby's blood type. Therefore it is impossible for her to be the mother of a type O baby.
