a b c d

a

$\text{Number of wave in a given length is } N = \frac{L}{\lambda} = \frac{L\mu}{\lambda_0}$ $ N_1 = N_2 $ $ \frac{L_1\mu_1}{\lambda_0} = \frac{L_2\mu_2}{\lambda_0}$ $\Rightarrow L_1\mu_1 = L_2\mu_2$ $\Rightarrow \mu_2 = \mu_1 \frac{L_1}{L_2} = \frac{5}{3}$