a. True. They describe the maximal operational settings that are permissible without damage to the anode and tube housing.
b. True. There are rating charts that display curves showing the maximum permissible mA depending on exposure time and peak kV (kVp) for a particular X-ray tube and focal spot size.
c. False. The size of the focal spot has an influence on the tube current that can safely be tolerated.
e. False. Ratings for repeated exposures are lower as heat accumulation needs to be taken into account.