Robert,
Maximum intensity means there is constructive interference at that point. That occurs when the extra distance is an integral multiple of the wavelength. Find the wavelength by lamda = speed of sound/frequency = 331 m/s/680 Hz = 0.487 m. That doesn't match any of the choices though. But multiply the wavelength by 3 (integral multiple) and you get 1.46 m, which to two sig figures is 1.50 m. It also has a max intensity at 0.487 m, 0.97 m, etc.
None of the other choices are integral multiples of the wavelength.
John
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