b
Here, $\lambda_{\mathrm{A}}=100 \mathrm{\,mm}=0.10 \mathrm{\,m}$
$\lambda_{\mathrm{B}}=0.25 \mathrm{\,m}$
$\mathrm{v}_{\mathrm{A}}=80 \mathrm{\,cms}^{-1}=0.80 \mathrm{\,ms}^{-1}$
As the frequency of the wave remains same in the two media
$\therefore v=\frac{v_{A}}{\lambda_{B}}=\frac{v_{B}}{\lambda_{B}}$
$\therefore v_{B}=\frac{v_{B}}{\lambda_{A}} \times v_{A}=\frac{0.25}{0.10} \times 0.80$
$\Rightarrow \mathrm{v}_{\mathrm{B}}=2 \mathrm{\,ms}^{-1}$