Concept: The question tests on the reasoning associated with Mean as a measure of Central Tendency.

Solution:

Average height of the students in class X(X) = Sum of the heights /number of students in class X

Average height of the students in class Y(Y) = Sum of the heights /number of students in class Y

We are asked Ratio = X/Y =?

 

St (1)  Given X = 120 but we have no information on Y.  (Insufficient)

St (2)   X+Y = 126 and since the heights can be non-integer value hence we cannot use this one equation to find Y.(Insufficient)

Combining both,

 X = 120cm
X+Y = 126cm
We DON'T KNOW the number of students in each class.
 By varying the number of students we shal have different values for Y.

Thus not sufficient. Hence Option(E)