Here's the scenario: There is a Private Pilot working on their instrument rating. Alongside with him is their CFII. Let's say it is just a normal VFR flight, and they're getting back to their home airport after a training flight. Now, after the first radio call, ATC instructs them "NXYZ, Squawk 1234 and ident - remain outside (Airport) Class Charlie".
Even though you just need Two-Way Radio Communications to enter Class C, ATC has strictly advised them to remain outside of it.
Now, for some reason, the airplane goes well inside Class C airspace. After a few minutes, ATC gives a call saying "NXYZ, cleared into Class C, enter left base Runway 18. Possible pilot deviation, advise when ready to copy down a phone number"
The question is - since both the Private Pilot and the Instructor are logging the flight time as PIC, who is responsible to answer? And if a deviation is given - who among the two gets it?