Under what conditions is passing on the right legal?

Passing on the right is legal under specific circumstances, and both situations listed— a 4-lane highway with two lanes going in each direction and a one-way street—meet those criteria.

On a 4-lane highway, where there are two lanes designated for traffic moving in the same direction, it is permissible to pass on the right when the vehicle in the lane to the left is making a left turn or is otherwise unable to move forward. This situation allows for safe passing as long as it doesn’t impede the flow of traffic.

On a one-way street, all lanes are moving in the same direction. In this case, passing on the right is also legal since there are no opposing vehicles to create a hazard; drivers can utilize any lane to overtake.

These points clarify why both a 4-lane highway and a one-way street enable legal passing on the right, making the combined answer correct.