What age must a supervising driver be for a learner's permit holder?

Answer: C — 21 years old

The requirement for a supervising driver is crucial for the safety and effective learning of a learner's permit holder. In Maryland, the supervising driver must be at least 21 years old and possess a valid driver's license. This age requirement ensures that the supervising driver has sufficient mature experience and a deeper understanding of the rules of the road, which is essential when guiding a novice driver. This age mandate is in place to create a safer learning environment that can positively influence the learner's driving skills and knowledge. Additionally, having a supervising driver who is at least 21 years old aligns with the state's emphasis on responsibility and safety during the training process, as a more experienced driver is more capable of providing appropriate guidance and handling potential roadside situations.