What must you be/have to supervise a learner driver?

Answer: B — Over 21 and have had a valid full license for more than 3 years

To supervise a learner driver, you must be over 21 and have held a valid full license for more than 3 years. Option A is incorrect because simply being over 18 and having valid insurance does not necessarily mean you have enough driving experience to supervise a learner driver. Option C is incorrect because while a professional driving instructor certificate would indicate qualifications and experience in teaching driving, it is not the only requirement for supervising a learner driver. Option D is incorrect because while having a vehicle with dual control would provide additional safety measures, it is not a requirement for supervising a learner driver. You must meet both age and driving experience criteria, as stated in option B, to be able to supervise a learner driver.