Read the following table :

Blood Group	Number of Students
A	9
B	6
O	12
AB	3
Total	30

What is the ratio of the most common and rarest blood groups ?

Correct Option: 4 : 1

To find the ratio of the most common blood group to the rarest blood group, we examine the frequency (number of students) for each blood group given in the table:

- Blood Group A: 9 students
- Blood Group B: 6 students
- Blood Group O: 12 students
- Blood Group AB: 3 students
- Total students = 30

From the data provided:

1. The most common blood group is the one with the maximum number of students, which is Blood Group O with 12 students.

2. The rarest blood group is the one with the minimum number of students, which is Blood Group AB with 3 students.

Now, we calculate the required ratio of the most common blood group to the rarest blood group:

$\text{Ratio}=\frac{\text{Number of students with Blood Group O}}{\text{Number of students with Blood Group AB}}$

$\text{Ratio}=\frac{12}{3}=\frac{4}{1}$

Thus, the ratio is 4 : 1.