Answer
(a) decrease
(b) II
Work Step by Step
(a) We know that $\lambda=\frac{h}{mv}$. This equation shows that wavelength is inversely proportional to speed and hence when the speed increases then the wavelength decreases.
(b) We know that the best explanation is option (II) -- that is, as the momentum of the car increases, it follows that the de Broglie wavelength will decrease, because it is inversely proportional to the wavelength.