The correct answer is: B. more than 50%.
A lifting machine is reversible if it can be used to lift a weight and then lower it back to its original position without any additional energy input. This means that the efficiency of the lifting machine must be greater than 50%, since the machine must be able to do more work than the amount of energy it consumes.
Option A is incorrect because a lifting machine with an efficiency of less than 50% would not be able to lift a weight and then lower it back to its original position without additional energy input.
Option C is incorrect because a lifting machine with an efficiency of exactly 66.67% would not be able to lift a weight and then lower it back to its original position without additional energy input.
Option D is incorrect because a lifting machine with an efficiency of 100% would be able to lift a weight and then lower it back to its original position without any additional energy input. However, this is not possible in practice, since there are always some losses due to friction and other factors.