'Green muffler' is a measure adopted to control

Concept: "Green Muffler" is an environmental-management technique in which dense rows of trees and shrubs are planted as a vegetative barrier. Leaves, branches and trunks absorb, deflect and scatter sound waves, so a thick green belt acts like a natural sound-dampening screen — the same role a "muffler" plays on a vehicle exhaust.

Application: Such belts are raised along highways, railway tracks, airports and industrial zones — exactly the high-decibel locations. A dense multi-row plantation can cut perceived noise by roughly 10-15 decibels, which is why the scheme is classed as a remedy for unwanted, excessive sound.

Contrast: The technique targets sound energy, not airborne gases, suspended water-borne contaminants, or contamination of land. Among the choices it therefore maps to the pollution category defined by excessive sound.

Result: "Green Muffler" is a measure adopted to control noise pollution.