The requirement for a supervising driver is that they must be at least 21 years old and possess a valid driver's license. This stipulation ensures that the supervising driver has sufficient experience and maturity to guide a learner driver safely. The age and licensing requirements reflect a balance of legal responsibility and driving competence, aimed at ensuring the safety of both the learner and others on the road. While a supervising driver needs to have experience, simply having a certain number of years of driving (as indicated in the last option) does not specifically address the age and licensing criteria, which are crucial for the supervisory role. Additionally, not all supervising drivers need to be licensed instructors, as many qualified individuals over the age of 21 may fulfill the requirements without being professional driving instructors. This underscores the focus on both age and validated driving capability, making it essential for a supervising driver to meet these specific conditions.

A solid white line on the roadway indicates separate lanes of traffic moving in the same direction. This line reinforces lane discipline, meaning that drivers should remain in their designated lane and avoid crossing the line unless necessary, such as to avoid an obstacle or during a legal maneuver. This distinction is important for maintaining smooth traffic flow and ensuring safety, as it helps prevent confusion and collisions by clearly defining where one lane ends and another begins for drivers traveling in the same direction. The other options suggest conditions or situations that do not apply to a solid white line. For instance, a solid white line does not indicate areas for passing, stopping, or separating opposing lanes; those require different markings, such as broken lines or specifically designated symbols.

Before changing lanes, it is essential to check mirrors and blind spots. This practice ensures that you are fully aware of your surroundings and any other vehicles or obstacles that may be in the adjacent lane. Checking mirrors gives you a general view of what's behind and to the sides of your vehicle, while also being vigilant in monitoring your blind spots—areas that are not visible through your mirrors. Using just your turn signal does not provide you with the information needed to gauge safety when changing lanes. While signaling is a crucial aspect of lane changes to communicate your intentions to other road users, it doesn't replace the necessity of visual checks. Looking back over your shoulder alone may not provide adequate peripheral awareness and can also distract from other important views, such as checking mirrors. Relying solely on the rearview mirror is insufficient as it may not show vehicles that are in your blind spots. Therefore, a combination of checking mirrors along with blind spot awareness is critical for safe lane changes.

The proper way to use a roundabout includes yielding to traffic already in the roundabout and signaling your exit. This approach is essential for maintaining a smooth flow of traffic and ensuring safety for all road users. Yielding means that when you approach a roundabout, you should slow down and give way to vehicles that are already circulating in the roundabout. This helps to avoid collisions and allows for an efficient transition into the roundabout. Signaling your exit is also crucial as it communicates your intentions to other drivers, which enhances predictability and reduces the risk of confusion or accidents. By using your turn signal to indicate when you plan to exit, you alert those around you, promoting safe driving practices. Using any other approach, such as speeding through without stopping, would disrupt the flow and increase the likelihood of accidents. Entering without looking ignores the responsibility of assessing traffic conditions, which is necessary for safe navigation. Stopping at the entrance can cause unnecessary congestion and potentially lead to rear-end collisions, especially if there are vehicles behind you expecting a continuous flow of traffic. Thus, option B is the most logical and safe choice when using a roundabout.