Correct Option: D

A node is a point along a standing wave where the wave has minimal amplitude. The opposite of a node is an anti-node, a point where the amplitude of the standing wave is a maximum. The distance between a node and anti-node is 1/2 of a wavelength; since a wavelength is from one node to another (or any point to its next “identical” point). So if w is the wavelength, then
1 2
= 30. So, w = 30 × 2 = 60 cm