Pretty sure it’s D. Could be B though
A semicolons NEVER should be used to try to create a "pause" longer than that achieved by a comma. If you can use a period correctly, then you can use a semicolon correctly. A semicolon is used just like a period, but it is used to separate two complete sentences when the sentences are closely related in meaning.
B. to inform, to persuade, and to inquire
I believe the answer is B. Nguyen explains how buses reduce traffic by lowering the number of cars on the road.
The passage says “Clearly, buses reduce traffic rather than cause it”. Answer B is the only answer that mentions traffic in a logical way.
I hope this helps!