When screw threads are to be used in a situation where power is being transmitted in one direction only, then the screw threads suitable for this will be

Answer: Option D

Buttress threads are best suited when power is transmitted in one direction only.

Reason: These threads have one face nearly perpendicular to the axis and the other face at an angle. This gives them high strength to transmit force efficiently in one direction, while reducing friction losses.

Other options:
- Knuckle threads are mainly used in couplings and linkages subjected to heavy jerks and vibrations, not for power transmission.

- Square threads are used for power transmission where efficiency is needed in both directions, such as in screw jacks and presses.

- Acme threads are easier to manufacture than square threads and are widely used in lead screws of lathes, where motion is required in both directions.

Conclusion: Therefore, buttress threads are most suitable when the load or power acts in one direction only.