What should be done immediately after witnessing an electrical shock?

When witnessing an electrical shock, the immediate priority is to ensure the safety of both the victim and the rescuer. Shutting off power to the structure is the correct action to take first, as this prevents any further electrical hazards that could lead to additional injuries or complicate the situation. Having the power turned off eliminates the risk of ongoing electric shock and allows for safer conditions for intervening.

Looking at the other options provides insight into why they are less appropriate in this scenario. While contacting authorities is important, it should be done after ensuring the environment is safe. Administering water to a victim could be extremely dangerous because water conducts electricity and might exacerbate the situation if the power is still on. Leaving the scene is not advisable, as the victim may need immediate assistance, and it is crucial to manage the situation until help arrives, provided it is safe to do so. Thus, shutting off the power is the essential first step to ensuring safety.