Answer
(a) decrease
(b) $\frac{3}{4}$
Work Step by Step
(a) We know that the wavelength is directly proportional to speed. Hence, if the wave speed decreases, then the wavelength will decrease as well.
(b) We can find the required change in the wavelength as
$\frac{\lambda^{\prime}}{\lambda}=\frac{\frac{1}{4}c/f}{c/f}$
$\implies \frac{\lambda^{\prime}}{\lambda}=\frac{3}{4}$
