putting $n = 340Hz, v = 340 m/s,$ the length of air column in the pipe can be
$l = \frac{{(2N - 1)340}}{{4 \times 340}} = \frac{{(2N - 1)}}{4}m = \frac{{(2N - 1) \times 100}}{4}cm$
For $N = 1, 2, 3, ... $ we get $l = 25 cm, 75 cm, 125 cm ...$
As the tube is only $120 cm$ long, length of air column after water is poured in it may be $25 cm$ or $75 cm$ only, $125 cm$ is not possible, the corresponding length of water column in the tube will be $(120 -25) cm = 95 cm$ or $(120 -75) cm = 45 cm. $
Thus minimum length of water column is $45 cm.$