Class \(X\). Let total student \(x\)
Class \(Y\). Let total student \(y\)

1) average height of Class \(X =120\) cntimeters
as no info of \(x, y\) as well as average height of class \(Y\). Options, A and D gone Insufficient. \(\Large{\color{red}\times}\)

2) Average height combined of Class \(X\) and \(Y\) is 126 centimeters
Now \(x\) and \(y\) are unknown , if they were known solution was possible but in this case B is gone. \(\Large{\color{red}\times}\)

Combining 1 and 2

Differential for Class \(X\) is 6 centimeters on lesser side as average height both class is 126
but again we don't know \(x\) and \(y\) so we can't find out average height of Class \(Y\)

Diff height \(X \cdot \) number of students \(X =\) Diff height \(Y \cdot\) number of students \(Y\)
\(6x=\) (Diff height \(Y\)) \(\cdot y\). too many unkowns C is gone. \(\Large{\color{red}\times}\)

Therefore, __E__