Final answer:
High beams should be used in open country driving on dimly lit highways for better visibility and spotting potential obstacles. They are not advisable to use during snow, rain, or fog as they can reflect off particles and worsen visibility. So, the correct answer is option a.
Step-by-step explanation:
A driver should use high beams when driving in the open country on dimly lit highways where there are no other vehicles nearby. High beams help to increase visibility further down the road, which is beneficial for spotting animals or other obstacles in rural areas.
High beams are not designed to be used in adverse weather conditions such as during a snow storm, rain storm, or when it is foggy, as they can reflect off the precipitation and further impair visibility. So, the correct answer is option a.