Non-reversible machine:
A machine having an efficiency less than 50% is known as a non-reversible machine. Let's understand why this is the correct answer.

Efficiency of a machine:
Efficiency is a measure of how effectively a machine converts input energy into useful output energy. It is calculated as the ratio of useful output energy to the input energy.

Efficiency = (Useful output energy / Input energy) * 100%

Reversible machine:
A reversible machine is one that can operate in both forward and reverse directions and can convert energy from one form to another with 100% efficiency. In a reversible machine, there are no energy losses, and all the input energy is converted into useful output energy.

Non-reversible machine:
A non-reversible machine is one that cannot operate in both forward and reverse directions with 100% efficiency. It experiences energy losses in the form of heat, friction, and other inefficiencies. As a result, the useful output energy is less than the input energy, leading to an efficiency of less than 100%.

Efficiency less than 50%:
When the efficiency of a machine is less than 50%, it means that more than half of the input energy is being lost and not converted into useful output energy. This indicates a significant energy loss and inefficiency in the machine's operation.

Example:
For example, if a machine has an efficiency of 40%, it means that only 40% of the input energy is being converted into useful output energy, while the remaining 60% is lost as heat or other forms of energy. This loss of energy reduces the overall efficiency of the machine.

Conclusion:
Therefore, a machine having an efficiency less than 50% is known as a non-reversible machine because it is unable to convert a significant portion of the input energy into useful output energy.