The correct answer is D. The other two bulbs will light.
When three 60 W bulbs are in parallel across the 60 V power line, each bulb will have a voltage of 60 V across it. If one bulb burns open, the other two bulbs will still have 60 V across them and will continue to light. The current in the main line will not change, as the total resistance of the circuit remains the same.
Option A is incorrect because there will not be heavy current in the main line. The current in the main line will be the same as before the bulb burned open, as the total resistance of the circuit remains the same.
Option B is incorrect because the rest of the two bulbs will light. The other two bulbs will still have 60 V across them and will continue to light.
Option C is incorrect because all three bulbs will not light. Only the other two bulbs will light, as the first bulb has burned open.