If father is blood gp A and mother is blood gp B then

what are the possibilities of offsprings?

It depends on the genotype of father and mother.
Genotype of father may be I(A) I(A) or I(A) I(O).
Similarly, Genotype of mother may be I(B) I(B) or I(B) I(O).
So child can have genotype-
I(A) I(B)-----> blood gp AB
I(A) I(O)-----> blood gp A
I(B) I(O)-----> blood gp B
I(O) I(O)-----> blood gp O.
Hope this helps.. :-)

AB, A , B, or O...
depends on genotypr
blood type is codominant so everything shows...

Could be A, B, AB or O.