Answer:
A, B or O
Step-by-step explanation:
The ABO blood type is regulated by three alleles of gene "I". The genotype "ii" gives blood type "O". Since the son of the given parents have blood type "O" (genotype: ii), both parents should have one copy of "i" allele.
Hence, the possible genotype and respective phenotypes for the father are:
1. IAi (A blood type)
2. IBi (B blood type)
3. ii (O blood type)