Entirely reasonable question, though once a PPL, the answer should be fairly clear in your mind. You want to understand what's not going to work any more, when a certain system quits in flight.
As said, it is the electric turn co-ordinator, as you realize with the relationship to the master switch. You will also see the small red flag in the window on the face of the turn co-ordinator, which tells you that it is getting electricity. In very old planes, particularly those which do not have an attitude indicator, or DG, you may have a "Turn and Bank" (upward white bar), as opposed to a "turn Co-ordinator" (little plane). Some turn and banks, are vacuum driven, rather than electric. They will therefore not start with the master.
When you turn the master off, shortly after the engine has stopped turning, you should expect that the electric turn co-ordinator will stop spinning first. The gyros in the attitude indicator, and DG (vacuum) should spin on much longer (many minutes). If they do not, expect that the offender is somewhat defective. The vacuum instruments have much more precise bearings and balance than the electric ones, thus should spin much longer. There is also the electric motor brush drag of the electric gyro to slow it down more quickly.
I hope that helps. (and by the way, you're done your 10 minutes of looking stupid- carry on with pride!)
Pilot DAR