Explanation

(c) The frequency of a wave is $v = \frac{v}{\lambda}$ where v is velocity and λ is wavelength.
For first wave,$v_{1} = 396\ ms^{- 1}$
$\lambda_{1} = 99cm = 99 \times 10^{- 2}m$
$\therefore v_{1} = \frac{396}{99} \times 100 = 400Hz$
$For\ second\ wavw,\ v_{2} = 396\ ms^{- 1}$
$\lambda_{2} = 100\ cm = 100 \times 10^{- 2}m$
$\therefore v_{2} = \frac{396}{100} \times 100\ Hz$
Number of beats = difference in frequencies.
$= v1 - v_{= 400} - 396 = 4$