Understanding Differential Mode Voltage of a Floating Circuit?
The secondary circuit is coupled to the primary supply by the magnetic flux across the transformer. They do not otherwise share a common reference to ground.
If you as a grounded person touch either C or alternatively D you will effectively become the ground reference which anchors the relative potential of the secondary circuit. That is the secondary circuit will no longer be floating.
Touching both C and D will give you a shock as there will clearly be a voltage between them irrespective of whether you are grounded or not.
Clearly the winner in this case is A as the primary circuit has a ground reference (is not floating) and neither are you (as you're grounded) so a shock is guaranteed.