Answer:
The correct answer is - 4/16.
Explanation:
In this question, it is stated that in ABO blood system the proper phenotypic expression of A, B, AB blood types needs H substance. If an individual has recessive for the H substance (hh) will have a type O phenotype.
The cross: ABHh and ABHh
gametes: two set of AH, BH, Ah, Bh
Punnett:
     AH     BH      Ah      Bh
AH   AAHH  ABHH   AAHh   ABHh
BH   ABHH  BBHH   ABHh   BBHh
Ah   AAHh   ABHh   AAhh    ABhh
Bh   ABHh   BBHh   ABhh    BBhh
As shown there is only 4 out of 16 offspring are recessive for H substance therefore, only 4/16 will have O blood group