Which of the following methods is least likely to enhance a motorcycle's torque?

Upgrading to a 2-stroke engine is the least likely method to enhance a motorcycle's torque when compared to the other options. While two-stroke engines can produce high power for their size due to their design, they typically deliver their power at higher RPMs. Torque, on the other hand, is often a product of how effectively the engine can operate at lower RPMs, where many motorcycles are designed to perform optimally.

In contrast, installing a performance air filter can improve air intake efficiency, allowing for better combustion and potentially increasing torque across the RPM range. Reducing the vehicle's weight can enhance acceleration and torque delivery as it decreases the load that the engine has to work against. Increasing the carburetor size can enable more fuel flow, which, when paired with sufficient air flow, can help enhance torque by facilitating better combustion conditions.

Therefore, while each of the other choices may contribute positively towards enhancing a motorcycle's torque, migrating to a two-stroke engine does not inherently guarantee a torque increase, making it the least effective option among the choices provided.