Why Current through tissue does damage in three distinct ways, so all of the listed effects are real. Resistive heating burns tissue along the current path (often worst at entry and exit points), the current interferes with the small bioelectric signals cells use, including the heart's rhythm, and it drives involuntary muscle contraction that can prevent you from letting go of the conductor. Even modest currents, on the order of 100 mA through the chest, can be fatal by causing ventricular fibrillation.
Watch out Each single-effect choice is true by itself but incomplete, so picking one of them misses the fact that the pool lists all three as valid hazards.
Current burns, confuses and clamps: heat, cell signals, muscle lock. Three hazards, so the all-of-these answer.