For least intense sound, $1 \,m$ can cause a destructive interference. So, path difference is an odd multiple of half wavelength.

$\Rightarrow \Delta L=(2 n+1) \frac{\lambda_1}{2}$

$\Rightarrow 1 \,m =\frac{\lambda_1, 3 \lambda_1, 5 \lambda_1}{2}, \frac{2}{2}, \text { etc. }$

Above condition is satisfied, when $\lambda_1=2 \,m$.

And for most intense sound, path difference of $1 m$ can cause a constructive interference. So, path difference must be a whole number multiple of one wavelength.

$\Rightarrow \Delta L=n \lambda_2$

$\Rightarrow 1 \,m =\lambda_2, 2 \lambda_2, 3 \lambda_2, \ldots, \text { etc. }$

Above condition is satisf ied with $\lambda_2=1 \,m$.

So, correct option is $(c)$.