Which of the following is most harmful to the human body?

The most harmful factor to the human body among the choices provided is current. When electricity flows through the body, it can disrupt normal physiological functions. The severity of the effect largely depends on the amount of current, measured in amperes. Even small currents can cause involuntary muscle contractions, while larger currents can lead to serious injuries or even fatalities, such as cardiac arrest or respiratory failure.

High voltage refers to the potential difference that drives the current through the body; however, it is the current itself that actually poses the risk of harm to biological tissues. High resistance usually indicates that less current will flow through an object or person for a given voltage, which can reduce the likelihood of injury. Reactance, related to AC circuits, also affects how current behaves but does not directly cause harm like current itself does.

In summary, the direct flow of current through the body is what can cause the most damage, making it the most harmful option listed.