Under which of the following circumstances is overtaking and passing to the right not permitted?

Overtaking and passing to the right is not permitted when doing so requires you to drive off the paved portion of the highway. This is because safety is the primary concern in driving; moving onto an unpaved surface can lead to loss of control of the vehicle and increase the risk of collisions or other accidents. Driving off the roadway to pass could also create hazardous situations for other drivers and pedestrians.

In contrast, passing on a one-way road is generally permitted because the configuration allows for driving on either side without the risk of encountering oncoming traffic. Overtaking a vehicle on the shoulder when it is turning left is permissible as well, provided it is safe to do so. Similarly, if a vehicle is proceeding significantly slower than the speed limit, passing on the right may be allowable, as long as it does not involve leaving the roadway. Therefore, option C identifies a specific and critical safety concern that makes overtaking to the right unacceptable in that scenario.