Final answer:
When the frequency of a photon is doubled, the wavelength is halved, according to the inverse relationship between frequency and wavelength when the speed of light is constant.
Step-by-step explanation:
If a photon's frequency is increased by a factor of two, the relationship between frequency and wavelength tells us that the wavelength is inversely proportional to the frequency. This is because the speed of light is a constant (c = approximately 3 x 108 meters per second), and the relation c = frequency x wavelength dictates that if the frequency increases, the wavelength must decrease to maintain the constant speed of light. Therefore, when the frequency of a photon is doubled, its wavelength is halved. So, the correct answer to the question is a) It is halved.