The correct option is C 6.25 cm
We know that ,
phase difference (Δϕ)=2πλ× path difference (Δx)
⇒Δx=λ2π(Δϕ)
But, v=fλ using this in the above equation we get ,
Δx=v2πf(Δϕ)
From the data given in the question,
Δx=3402×π×680×π4=116=0.0625 m or 6.25 cm
Thus, option (c) is the correct answer.