The correct answer is: A. to designate traffic lanes.
Centre line markings are used to separate traffic lanes on a roadway. They can be solid or dashed, and they can be white or yellow. Solid centre lines indicate that drivers should not cross the line, while dashed centre lines indicate that drivers may cross the line to pass or turn.
Option B is incorrect because centre line markings are not only used in roadways meant for two-way traffic. They can also be used in one-way streets and in divided highways.
Option C is incorrect because centre line markings do not always indicate that overtaking is not permitted. In some cases, overtaking is allowed when the centre line is dashed.
Option D is incorrect because centre line markings do not designate proper lateral placement of vehicles before turning to different directions. This is typically done by using turn lanes or by marking the roadway with arrows.