Mrs. Brown and Mr. Brown offspring most likely to be blood type B according to Punnett Square.
Mrs. Brown have blood type IB i (B)
Mr. Brown have blood type IB IA (AB)
The Punnett results for the offspring are:
B/ AB/ B / A (2/4 to type B)
(1/4 to type A)
(1/4 to type AB)