An ideal transformer connects a 120-V, 60-Hz voltage source to a load. One terminal of the voltage source is grounded. The load is ungrounded. The circuit and transformer are operating normally (not in a fault condition). A person whose body is contacting ground is in proximity to the circuit. If the person touches the circuit, which point (A, B, C, or D) likely has the highest risk of resulting in a shock?

A.  Point A
B.  Point B
C.  Point C
D.  Point D

If a person whose body is contacting ground is in proximity to the circuit, then touching point A would provide the highest likelihood of resulting in a shock. 

To explain further, a shock occurs when current travels from a source of high potential (120V in our example) to a source of low potential (GND). 

Since the person's body is also contacting ground, we can treat the person as essentially a resistor to ground.

Point B is connected to GND. Therefore, the potential is the same between the person and Point B. No current will flow.

Even though there is a voltage across the secondary side of the transformer, we are told that this side is not grounded. Therefore, there is no way to keep this voltage at a stable value relative to a reference, because this reference (GND) is not present. When touching points C or D, the potential at those points will immediately align with the potential of the person. Without a difference of potential, there is no risk of shock. Since a difference of potential exists at some point at the very beginning, there is still some risk involved, but it is low.

Point A has a high potential, however, and the primary side is grounded. Therefore, even when touching point A, the potential will remain the same (in this case 120V). This difference of potential will cause current to flow and result in a shock.