Final answer:
Ignition system wiring, electrical power lines, and neon signs can all serve as sources of radio interference, hence the correct answer is 'd. All of the above'.
Step-by-step explanation:
Sources of radio interference can come from a variety of sources. The options given in the question include ignition system wiring, electrical power lines, and neon signs. Each of these can indeed be a source of electromagnetic disturbances that interfere with radio signals. Ignition systems in vehicles can produce interference due to the sparking process that takes place. Electrical power lines can emit interference from the high voltage they carry and especially during instances like switching operations or faults. Lastly, neon signs, which operate by exciting neon atoms to emit light, can also create interference in the radio-wave frequencies due to their electrical operating nature.
The correct answer to the question on which of the following can be sources of radio interference is: d. All of the above.