Under what condition is an LED lit?

An LED lights up when it is forward-biased, meaning that it must be connected in such a way that the positive terminal of the voltage source is connected to the anode (positive side) and the negative terminal is connected to the cathode (negative side). This allows current to flow through the diode, which generates light as electrons recombine with holes within the semiconductor material.

The other conditions do not facilitate the lighting of the LED. When the arrows point to the negative terminal, it suggests a reverse bias situation, which would not allow current to flow through the LED, keeping it unlit. Disconnecting the LED from the circuit would also mean no current can flow, and thus it would not light. Lastly, while exceeding the voltage rating can cause the LED to light up under certain circumstances, it's important for the voltage to be within the appropriate limits to avoid damage to the LED.