Answer
(a) always has a longer wavelength than the incident photon.
Work Step by Step
Compton assumed that photon behaves as particle when it collides with the electron. Assuming it to be the elastic collision between electron at rest and incident photon some energy redistribution takes place between electron and photon.
photon transfers some of its energy and momentum to the electron. Energy of photon is given by E = hf, where f is frequency of photon. so decrease in energy of photon means decrease in its frequency.
Frequency is inversely proportional to wavelength so after scattering we will have longer wavelength photon.
