Explanation

(a) Using relation v=v$\lambda$
Or
$\lambda = \frac{v}{v} = \frac{340}{340} - 1m$
If length of resonance columns are $l_{1},l_{2}and\ l_{3},$ then
$l_{1} = \frac{\lambda}{4} = \frac{1}{4}m = 25\ cm\ \ (for\ first\ resonance)$
$l_{2} = \frac{3\lambda}{4} = 75cm\ \ \ (for\ second\ resonance)$
$l_{3} = \frac{5\lambda}{4} = 125\ cm\ \ \ \ for\ third\ resonance$
This case of third resonance is impossible because total length of the tube is 120 cm
So, minimum height of water =120-75=45cm