At terminal velocity, the speed of an object falling through a fluid (like air) depends on its weight and the drag force acting on it. Generally, objects with larger surface areas experience greater air resistance and therefore have slower terminal velocities.
Among the options you've provided, the object likely to have the slowest speed at terminal velocity would be "a 50-kilogram block with a large parachute." The large parachute significantly increases the drag force, lowering the terminal velocity compared to the other objects listed.