A vehicle turning left at an intersection must __________ to oncoming traffic.

When a vehicle is turning left at an intersection, it is essential for the driver to yield to oncoming traffic. This is necessary to ensure safety for all road users, as oncoming vehicles could be moving straight through the intersection. Yielding means the driver must wait for a safe opportunity to make the turn without interfering with or endangering the approach of other vehicles.

In addition, this practice follows traffic laws which prioritize the right of way, especially at intersections where multiple directions of traffic converge. By yielding, the driver shows that they are aware of their surroundings and are taking the necessary precautions to avoid collisions. The other options are not appropriate in this context; for instance, stopping may not be necessary if the way is clear, merging does not apply as the vehicle is not entering a lane next to another vehicle, and accelerating would increase the risk of a crash. Therefore, yielding is the correct and safest action to take when turning left at an intersection.