Answer is A, Don r is correct, as is Robert.
If the piston ring grooves are worn to the point they allow the rings to rock around, the pistons are unserviceable. The answer is "A".
Oversize piston rings would be needed if you honed the cylinder out a little bigger than spec, but not enough to need oversize pistons. The cylinder size would determine this, not the piston ring groove.
Piston-to-cylinder clearance is the difference between the largest outside diameter of the piston, and the cylinder diameter. The ring groove diameter is considerable smaller than the OD of the cylinder, and would not affect this measurement either.
If the ring grooves were damaged or cracked, they may damage the rings when installed, or may break off of the piston, causing damage when you start the engine. So the correct answer is A- to determine if the piston is serviceable.
Gaps - To determine if you need oversize rings on a rebuild. If the ring groves are worn you need new pistons.
I don't know
B mainly but all three sound applicable
