In a stationary wave pattern set up in a string of length 1.2 m, three antinodes are observed. If the frequency of vibration is 150 Hz, what is the speed of the wave in the string?

Three antinodes in a standing wave on a string fixed at both ends correspond to the third harmonic, meaning the string contains 3 half-wavelengths: $L = \frac{3\lambda}{2}$, so $\lambda = \frac{2L}{3} = \frac{2 \times 1.2}{3} = 0.8$ m. The wave speed is $v = f\lambda = 150 \times 0.8 = 120$ m/s. Option A results from using one full wavelength equal to the string length, while C and D arise from other incorrect harmonic assumptions.