How old must you be to legally supervise a learner driver?

Answer: B — 21 years

To legally supervise a learner driver in the UK, one must be at least 21 years old. This requirement ensures that the supervising driver has sufficient life experience and maturity to effectively assist and guide the learner in their development of driving skills. Additionally, the supervisor must have held a full driving licence for a minimum of three years for the category of vehicle being driven. The other age options do not meet the minimum legal age requirement for a supervising driver, which is why they are not correct in this context.