What must you be aware of here?

In the context of the traffic rules related to the options provided:

1. Stopping to the left of the boundary line of the carriageway is not allowed to ensure safety and smooth traffic flow.
2. Slower-moving vehicles are required to use the hard shoulder, if available, to prevent obstructions in the main flow of traffic. This promotes a better driving environment by allowing faster vehicles to pass without delay.

The third option, which states that driving across the boundary line is always prohibited, is more nuanced. While reckless or inappropriate driving across the line can lead to hazards, there are circumstances under which crossing may be allowed (e.g., for turning or merging), making it necessary to focus on the other two points for strict adherence to safety.

Understanding these rules helps maintain road safety and efficient traffic management.