Interference Between Power and Instrument Cables

Don't shield or separate the conductors. That'll give you plenty of interference.

Cross talk, poor shielding or lack of separation.

I hope you're not taking a test.

See Radio frequency interference (RFI)...

http://www.spiraxsarco.com/resources/steam-engineering-tutorials/control-applications/control-installations.asp

The "reason" for interference would be improper cable selection by an inexperienced designer.

You must have studied something called "proximity effecet". Whenever a conductor carries a current, a magnetic field (flux) is et up around the conductor. This flux will cut any nearby conductor and as per the famous Faraday's law, will induce an EMF in the nearby conductor (provided that it is an alternating flux). If the nearby conductor has a closed path, then there will be an induced current flow due to this induced EMF. This induced current will interfere with the original current that was/is flowing in the nearby conductor. This is particularly problematic, if the nearby conductor happens to be the part of an Instrumentation Signal cable, wherein the signal is alreday very weak; the interference thus causing data errors or worse.