You may only enter a box junction when:

Entering a box junction is permitted only when the exit road is clear. This regulation is designed to prevent congestion and ensure the flow of traffic. When approaching a box junction, if the exit is blocked, entering the junction could result in obstruction for vehicles in other lanes and could lead to gridlock or accidents. By ensuring that the exit is clear before entering the box junction, you contribute to the smooth operation of the intersection and help maintain a steady traffic flow.

The other choices revolve around conditions that do not guarantee safe entry into the box junction. For instance, if there’s no traffic around, you may find it tempting to enter even when the exit isn't clear, which would still violate the rules. Similarly, the presence of stopped vehicles or smoothly moving traffic doesn't necessarily indicate that your exit is accessible. Only by confirming that your exit road is clear can you ensure compliance with traffic regulations while promoting safety for all road users.