Blood Group Ratio

[JackBean]

It's little bit hard to explain Punnet here.
First, you need to determine genotype of your parents. Say one will be AA and the other one Aa.
Then determine gametes they form. First parent will have only gametes containing A, while the second one will have gametes, which contain either A or a.
Then draw the Punnet square:
.....A...A
A
a
so, on top row are gametes from first parent, on first column are gametes from second parent.
Now just combine the numbers, which you have there:
.....A...A
A..AA...AA
a..Aa...Aa

[chikis]

I must thank everyone for their immense contribution to this very post.
To all those who sugested the use of punnet square; I must state categorically that punnet square has no application in the answering of this question.
The simple thing, I think should be done is to study the inheritance of blood groups, based on the study of thousands of families.
From my study, I notice that parents having blood group A and O will give birth to offspring with blood group O and A only. So the probable ratio of the blood group of the children will be 1:1 only. So the right answer is option A in the question.
Any body who is in doubt or wants to know more should go and study the use of blood group in determination of parentage. Thank you.

[JackBean]

of course the Punnet square is related, how else would you determine the ratio?

[chikis]

Is not related in anyway. You can determine the ratio by simply looking at the possible antigen A, O of the future ofspring. You can now see clearly that they are only two antigens.
Suppose the couple gave birth to only two children and they happen to share the two antigens equally among themself; that means the sharing will be in the ratio 1:1. With this explaination it is easy to see that I or you do not need punnet square to determine the ratio of the blood group of the children.