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