Final answer:
Option 4: More mass transit systems can lead to the most significant decrease of traffic in the city.
Step-by-step explanation:
Out of the four options presented, Option 4: Expanding mass transit networks is the transportation infrastructure modification that could result in the biggest reduction in traffic in the city. Mass transit vehicles, like buses, trains, and trams, can move a lot of people quickly and can contribute to a decrease in the number of cars on the road. Mass transit can encourage more people to use public transportation by offering a dependable and practical substitute for driving, which will lessen traffic congestion in the city.