The correct answer is: More than 50 lakhs.
A serious accident is an accident in which there is loss of railway property of more than 50 lakhs. This includes accidents that result in the derailment of a train, the collision of two trains, or the fire or explosion of a train.
The other options are incorrect because they do not meet the definition of a serious accident. Option A is incorrect because it does not specify the type of loss. Option B is incorrect because it is less than the amount of loss required for an accident to be considered serious. Option C is incorrect because it is less than the amount of loss required for an accident to be considered serious. Option D is incorrect because it is more than the amount of loss required for an accident to be considered serious.
I hope this helps!