Answer
(a) decrease
(b) option II
Work Step by Step
(a) We know that $f_{\circ}=\frac{W_{\circ}}{h}$. This equation shows that the energy of a photon is directly proportional to its frequency, but the frequency is inversely proportional to the wavelength. Thus, to increase the energy of the photons so that they can eject electrons, we need to decrease their wavelength.
(b) We know that the second option is correct -- that is, the energy of a photon is proportional to its frequency, which is inversely proportional to its wavelength. To increase the energy of the photons, one must decrease their wavelength.
