Step 1: Analyze the passage.
The passage highlights multiple benefits of public transportation systems. These include reducing traffic congestion, minimizing environmental impact, and contributing to lower levels of air pollution and greenhouse gas emissions.
Step 2: Focus on the key detail.
The specific line, "Cities with extensive public transit networks often experience lower levels of air pollution," directly supports option (C) — Reduced air pollution.
Step 3: Evaluate other option.
(A) Increased traffic congestion — The passage states the opposite: public transport helps reduce congestion.
(B) Higher greenhouse gas emissions — Again, the passage says emissions are reduced.
(D) Decreased urban development — The passage suggests public transport promotes more sustainable urban development, not a decrease.
Therefore, the correct answer is option (C) Reduced air pollution.