**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)