The correct answer is A. formal.
You need to use the formal level of diction when it comes to formal situations, such as writing scholarly works and giving speeches. The appropriate level of diction needs to be used to suit the particular situation. If you are talking to your friends, you will use the informal style. If you are talking to your teachers, you will use the formal style. Both formal and informal levels are standard, and there is no such level as a practiced level.