Answer
Choice C.
Work Step by Step
The movement of one speaker by 0.34 m turned a point of constructive interference (where a crest from one speaker met a crest from the other speaker) into one of destructive interference (where a crest now meets a trough). The distance moved was half of a wavelength.
$$0.34m=\frac{\lambda}{2}$$
$$\lambda=0.68m$$
Now find the frequency.
$$f=\frac{v}{\lambda}=\frac{340m/s}{0.68m}=500 Hz$$