The correct option is A 45 cm
We know that v=fλ
⇒λ=vf=340340=1 m
For a closed organ pipe (organ pipe filled with water), it's fundamental resonating length,
l1=λ4=14 m=25 cm
Second resonating length,
l2=3λ4=75 cm
Third resonating length,
l3=5λ4=125 cm
Third resonance is not possible because the length of tube is only 120 cm.
∴ Minimum height of water necessary for resonance,
Hmin=120−75=45 cm
Why this question?Tips: The free surface of water will form the closedend of organ pipe. Resonating length is correspondingto standing wave formation.