In mechanical systems, especially in the realm of theoretical and practical machine operations, the concept of work input and work output is fundamental. Here is the step-by-step explanation of why the correct answer is "always less than":

1. Understanding Work Input and Work Output:
   • Work input refers to the energy supplied to a machine to perform a task.
   • Work output is the useful work done by the machine when it uses the supplied energy.
2. Role of Friction:
   • Friction is a resistive force that acts opposite to the direction of motion.
   • It converts some amount of input energy into heat, which is not useful for performing work.
3. Efficiency and Energy Loss:
   • Efficiency (\(\eta\)) of a machine is defined as the ratio of work output to work input, expressed as a percentage:
   • \(\eta = \frac{\text{Work Output}}{\text{Work Input}} \times 100\%\)
   • Due to friction and other resistive forces, not all input energy is converted into useful output, causing efficiency to always be less than 100%.
4. Conclusion:
   • Since a portion of input work is always lost to friction, the work output of a machine is always less than the work input.

Thus, the correct answer is "always less than" due to the inevitable loss of some energy to friction in real-world machines.