Only the towing vehicle of a semi-trailer unit is equipped with an anti-lock braking system (ABS). What could the effect of this be when performing an emergency braking manoeuvre on a wet carriageway?

You answered correctly. When only the towing vehicle (tractor unit) has ABS and the trailer does not, during emergency braking on a wet road, the ABS will prevent the towing vehicle's wheels from locking up, but the trailer’s wheels could still lock. This can extend the braking distance (answer 1) because the trailer may slide instead of helping to stop efficiently. It can also cause the trailer to jack-knife (answer 2), which means the trailer swings out dangerously because its wheels have lost traction. ABS on both vehicles increases safety and control; if only one part of the combination has it, stability and braking performance are reduced. For similar questions, remember: ABS must be on all vehicle parts in a unit to get full braking control and stability.