#2 (Type B):

    (1) First, note that a type "A" Mother must be AA or AO:
            but, an AA mother cannot have either a type "B" child
 

Mother
Father
Children
AA
AO or AA
A only
AA
BO or BB
A or AB
AA
AB
A or AB
AA
OO
A only

    (2) So, if Mother must be AO:
 

Mother
Father
Children
AO
BB
1/2 AB, 1/2 B
AO
BO
1/4 A, 1/4 B, 1/4 AB, 1/4 O

#2 is included BUT
    ~12% of population has same blood type: "reasonable doubt"